THE RESPONSE OF ROOTS TO "ROOT­
FORM ING" SUBSTANCES P. W.
ZIMMERMAN AND
A. E.
HITCHCOCK
Several report s from this laboratory have concerned the responses of
stems and leaves of plants to t he so-call ed "hormones" or flgrowth sub­
stances." F or example, syntheti c chemical compou nds were shown to
induce epinasty of leaves , proliferations, swelliuR, bending an d r etarda ­
ti on in elongation of stems, a nd initiatio n of roots on stem s and (eaves
(1, 5) ' Roots were not tested because they were more or less inaccessible
Jor t his ty pe of experimental wo rk. OU f needs have recently been supplied
by a grape vine which produces aerial roots when grown in the greenho us e.
This report therefore con cerns the response of roots to growth su bstances.
The following synthetic compou nds were used in the t ests: (X~ naph ­
th aleneaceti c acid , indolebutyric acid, indoleacetic acid, indole propionic
acid, L'l-(3-indolyl)-valeric acid.' and phenylacetic acid. The concentration
ran ge when dissolved in water was 0.00007 per cent to 0.0006 per cent ;
as lanolin preparations the concentration range was o.or per cent to 1. 0
per cent. The roots were immersed in th e water solutions, but the lan olin
preparations were applied with a glass rod to the tips or a lon g t he side of
th e roots.
There is an unnamed species' of tropical gra pe (Vitis sp.) (3) which,
wh en grown in a greenho us e, climbs to the roof and then spreads out ,
clinging to girders a nd other objects which th e tendrils can grasp. Ae rial
ro ots gro w from the nod es of t he vines at frequent intervals and extend
great distances witho ut branching until they come in contact with soil or
oth er moist material. In th e Misso uri Botanical Ga rden Conservatory,
th e roots grow for distances of 30 to 40 feet with o ut bra nching (Fig. I ).
According to Moore (3), who s uppli ed cuttin gs for this la boratory , "the
internal stru cture of th e s ma ll roots is jus t like a ny other s mall root ."
At the Boyce Thompson Institute a plant set in t wo cubic feet of soil
grew vigorously, producing v ines more than 20 feet in length . As these
clu ng to th e r oof th ey p roduced aerial roots which soon tou ched the green­
house beds ten feet below. No branching of the roots occnrred until th e
tips came in contact wi th m oist material, su ch as s ail or water. Roots
which were immersed in water produced severa l bra nches which in turn
also bran ched if l eft for several days in water. When roots were cut off,
one o r two branches arose from th e remaining stumps. Frequently more
than one started bu t only one conti nued.
I All t he indole compounds used in these experiments were sy nthesized a nd furni shed
by Dr. R H. Manske of the National Research Council, Ottawa , Canada .
2 This plant is now ~cnown to be CiSS1tS sicyo£des L. vaL Jacguini Planchon.
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CONTRIBUTIO NS FROM BOYCE Tn OMPSON I NS TITUTE
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FIGURE I. Tropical grape vine in the Cycad houseatMissou ri notanical Garden show ing
aerial roots 40 feet long with out branching uotil they come in contact with the groun d .
(Courtesy Missouri Botanical Ga rden.)
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ZIMMERMAN
&
HITCHCOCK-"RoOT-FORMING" SUBSTANCES
44I
The average elongation per day of ten measured roots was four inches.
The g reatest elongation for anyone root was six inches. The region of
elongation as shown by increase in distance between millimeter marks
stamped along the side of the root extended IO or more inches back of the
tip. The region of greatest elongation extended from the tip back two to
three inches. For severed roots the elongation decreased as the distance
back of t he tip, wh ere the cut was made, increased. Roots with only one­
quarter inch of the tip removed mad e practicatly as much elongation
during the first 24 hours as intp.ct roots, while roots with six inches re­
moved showed very littl e elongation. The youn g part of the root was very
flexible while the older part was easily broken. The diameter which varied
around o ne-sixteenth of a n inch was nearly uniform throughout th e entire
length of a root.
The first noticeable responses of roots treated with lanolin preparations
of g rowth substances over the region of elongation were bending, retarda­
tion in extension, and increase in diameter. These are similar to the
response of etiolated stems of sweet pea and bean seedlings previously
reported (5). Within three days after treatment newly initiated branch
roots could be seen emerging through the epidermis . These continued to
elongate until th ey were approximately one-eighth inch in length , at which
time the tip of th e main root appeared to recover from the treatment and
renewed its growth. As soon as the rate of growth at the tip approached
normal the new side roots were completely inhibited. They w~re, however,
released again if the tip of the main root was re-treated with growth s ub­
stance. After they reached one-half i1)ch or more in length they appeared
to be freed from fur ther influe nce of the main root tip and went ahead to
elongate as norma l roots (Fi g. 2 A).
The effective concentrations for induci:ng roots ranged for the six
s ubst a nces from o. I per cent to 1.0 per cent when dissolved in lanolin and
applied along the region of elongation. Alpha -naphthaleneacetic acid
was toxic at 1.0 per cent, but was very effective at 0.05 to 0.5 per cent.
Phenylacetic acid, indolepropionic, and indolevaleric acid were only
slightly active at 0.2 per cent but were very effective when 1.0 per cent
was used . Only one application was necessary for inducing roots if th e
concentration was 0.2 p er cent or more of naphth aleneacetic acid , indole­
butyric acid, o r indoleacetic acid. Higher concentrations- approximately
0·5 per cent-were necessary for the other substances. The time necessary
for resumption of grow th at the tip varied with the concentration, th e
quantity of the preparation used and the nearness of application to the
point of the root. If 1.0 per cent indolebu tyric acid was applied over the
region of elongatio n even with, but n.ot over the tip, growth was resumed
within six to eight days. Recovery came earlier with indoleacetic than
naphthaleneacetic acid. E longation was greatly retarded, but not stopped,
with 0, I per cent or" th ese substances. When roots were initiated with low
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FIGU RE 2. Response of aeria l roots of Vit-is sp. to root-formin g substances. (A) Treat­
mell ts wit h lanolin preparations of na phthal eneacetic a cid. Left to right: (I ) control; (2)
response to excised tip; (3) treated with 0.1 per cent over reg ion of elongation; ti p recovered ,
ret a rding new root s; (4) treated with 0.5 of J per cen t; tip beginning to r ecove r ; (5) treated
ove r region of elongation w it h 0.5 of 1 pe r cent a nd after slight recovery, t ip was re-treated ,
t hereby preventing apical do minance; (6) trea ted with 0.5 of I per cen t a t tip only , ca nsing
swellin g; (7) initiation a nd growth of roots after tip only was treated a nd re-treated to pre­
vent recovery ; (8) g rowth of roots a fter tip had been treated, th en re· t reated, t.h ereby re·
leas ing 3 young root s which now d omin ate oth er roots. PhotoRraph taken 12 days after
firs t t reatmeut. (B) E ffect of imm ers ing roots in solution s of uap hthaleneacetic ac id. Left"
to right: (I) con trol in aix ; (2 ) con tr ol in tap water; (3), (4) , (5), a nd (6) in wa te r solutions
of naphthal eneacetic acid . 0.00 0 6 , 0.0003 , 0.000 1 5 , 0.000 0 7 per ce ut respect.ively. Ph oto­
gTaphed a fter 6 days in solution .
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ZIMME RMAN
& HITCH COCK- "RoOT-FORMING" SUBSTANCES
443
concentra tions they ma de onl y slight elongation before being inhibited as
a result of gro wth of the ma in root. W hen immersed in water over these
ret a rded root s, the influence from t he growing t ip disappeared. Also, if
the region produ cing t he new roots was immersed in water, the tip remain­
ing Qut of the water , the re was no apparen t inhibiting influen ce.
A la nolin prepa ration containing 0.5 per cent of t he growth snbstances
s meared over t he tip of a root caused considerable swell ing and, wit hin
five days, initiation of roots just back of t he treated part . As wit h t he
o ther tr eatments t he new roots were inhi bited as soo n as t he ma in root
r enewed its g rowth . W ith one o r two fe -treatments, however, t he new
r oots grew enough to be fr eed from t he influe nce o f t he main t ip (Fig. 2 A).
W hen ma ny roots we re initiated over a two-inc h zone and t hen the t ip
injured or cut off , n ot a ll of t he new roots continued to grow. There ap­
peared to be even competition for a short t ime, then two or three roots
gained an advan tage and therea fter a ll others ceased to e longate (F ig. 2 A).
Application of the growt h substances back o f t he region of elongation
was comparatively ineffective, This might have been d ue to an impervions
layer of co llenchyma snch as described by T urner (4) for roots of Vitis
rotundifolia. He believes those roots to be simila r to the ones of t he un­
named species used in these experiments. \iV hen t he s urface o f t he root
was scraped wi th a knife and the substance applied to t he injured a rea a
fe w roots were initiated in the adjacent regions. This type of response
su ppor ts the conclnsions of Tnrner.
\Alhen the ends of roots were immersed in solutions of growth snb­
stances, an abnormal thickening and a large number of new roots occurred ,
as compared with si milar roots in tap water . Though many new r oo ts were
init iated in a solution of 0.0006 per cent naphthaleneacetic acid, very little
growth was made. As the concentratiou was reduced t here wa s less re­
tardation as shown in Fignre 2 B. A concentration of 0.00007 p er cent
initiated many luore roots than the water control and there was practically
no retardation. Roots wh ich were indu ced in t he wa ter solntions of growth
substances grew in rows a long the region of elongation. The indications
were that the ceUs formi ng the roots were associated with longitndinal
strands of tissue.
As mentioned earlier the grape vines normally produce aerial roots
from th e nodes at freqnent inte rvals. Seldom more than one root per node
was found and never were roots p roduced on internodes of normal vines.
The same substances which ini tiated roots on aerial roots were also effec­
tive on the stems. When app lied at nodes or internodes many roots arose
from t he t reated region. The substances were most effective on portions
o f the vine t h at were within the region of elongation. Roots were best
induced fo ur to six nod es back of t he gr owing tip . In this resPect t h e re­
sponse of vines d iffers from that of roots . N evertheless, the same co m­
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CONTRIB UTI ONS FROM B OYCE TH OMPSON I NSTITUT E
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pounds induced roots o n both stems a nd roots. They must t herefore be
true root-inducing substances.
There are two points of special in terest in the results presen ted in this
paper: first, that growth substances induce new roots to form o n roots;
second, that the growing tip exhibi ts a marked dominance o ver newly
initiated branch roots but loses this power when it is artificia ll y retarded
with syn thetic g rowth s ubstances or immersed in water. In contrast with
t he latter response, w he n stem tips were treated with growth s ubstances
t he latera l buds were inhibited (2) .
Since all of t he che mical compo und s mentioned in conn ection with
these experiments have the capacity to induce roots to form on stems ,
leaves, and on roots t hemselves, it seems reasonable to call such agents
"root-forming substances." As to wheth er t hey act directly o r indirectly
is of little concern at this time. The evidence at hand seems to favor direct
action . When the t ip only is treated, new roots form immediately back of
this point; treated a long the region of elo ngation, the roots form o nly
there; if a zone eigh t inches back of the tip was treated, elo ngation was
retarded and roots fo rmed a ll the way dow n. T he indicati ons were t hat
t he substan ces moved more readil y toward t he tip than otherwise. H ow­
ever, elongation was greatly retarded when on ly the tip was treated.
ft would be of interest to know exactly why the aerial roots of Vh,:s
d o not branch unti l they strike soil, Or water, or are given special treat­
ment with chemical compouuds. This seems to point to a very strong
ap ical dominance which may mean the production of a snbstance by the
growing tip, exe rtin g a n influence bac k a long the root. If so, s uch age nts
of plants resemble the c he mical messengers of a nima ls commonl y called
" ho rmones."
It is significan t t hat t he tip must be in an act ive state of growth to
dominate branch roots. In fact, these branch roots can grow only when
t he tip is retarded or interfered with by som e means.
A question arises as to what becOInes of the powe rful domin ati ng age nt
when the growing part of the root is immersed in water. If the factor is a
chemical substance it might, of course, leach o ut of the tissue into the
water. To d o so, however, calls for a cha nge in direction of movement.
Anothe r view might be t hat under such conditio ns the root abso rbs water
a nd so dilutes the domi nat ing substance t hat it is no longer e ffective.
Wh ile g rowing in a ir a ll food materials and water must be sent to th e tip
from the vine. \\lh en t he root reaches th e soil it takes up a new function,
that of supplying water and mineral elements to the vine. The root , t here­
fo re, assumes its true function and th er e must begin an active fl ow of
materials through t he xylem. It is conceivable, t herefore, that any specia l
materials manufactured at the growing tip could get into the transpiration
strea m a nd be ca rried back to th e vine.
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ZIMMERMAN
& HITCHCOCK-"RoOT-FORMJNG" SUBSTANCES
445
Another theory, t hat of competition for food, was sugges ted as a
possibl e cause for failure of some branch roots to grow. That does not
seem possible for two reaso ns. First , ro ots nearest the so urce of food
supply are ofte n the o nes most retarded ; second, when t he root is im­
mersed in water a ll roots grow , tho ug h the same competition should main­
tain .
SUMMARY
I. An unnamed species of tropical grape (Viti.\' sp.) growing in a green­
house produced aerial roots w hich did not branch until com ing in contact
wit h soil or other moist material. If t he root was severed, o ne or two new
roots arose jus t above the cut s urface. The roots normally elongated an
ave rage of four in ches pe r day.
2. A study was made of the roo t-forming power of six chemical grow th
substances w hen app lied to roots, The substances, a-naphth aleneacetic
acid, iudolebutyric acid , indoleacetic acid, indolepropionic ac id, n- (3­
indolyl)-valeric acid, and pheuylacetic acid , were used as lan olin prepara ­
tions and as ..v ater solution s.
3. "\Then lanolin preparations of the s ubstances were applied along the
regio n of elongation new branch roots a ppeared through the epidermis in
thr ee to five days. The s ubs tan ces also caused swellin g and retardation
in elongation of the roots.
4. The g rowing tip was found to have a dominating influ ence over
branch roots. The n ewly ini tiated roots were inhibited \vh en the tip of th e
main root resnmed growth after having been retard ed by the substance
app lied to it. By re-treat ment of th e tip th e new branch roots could be
induced to grow. Also , if the region producing th e new roots we re immersed
in wate r the apical dominance was ove rco me.
s. Application of the grow th s ubstances back o f the region of elonga­
tio n was comparativel y iueffec tive. Eit her the material did not penetrate
or old tissue is not as susceptible a s that of the growing regIOn.
LITERATURE CITED
r. HItCHCOCK , A. E. Indole-3-n-propionic acid as a growth hormone and the quantitative
measurement of p lant response. Contrib. Boyce Th ompson Inst. 7: 87- 95· 193j.
- Tobacco as a test plan t for com par in g the effectiveness of prepa rations con­
taini ng grow th substan ces. Contrib. Boyce Thompson Ins t. 7: 349-364. 19J5.
J . MooR.E, JOHN AOAM. Remarkable aerial roots of a tropical grape v ine. Missouri Bot.
Card. Bull. 21: 139-140. 1933.
4. TURNER, Lm\O"I s !VI. Anato my of aerial roots of Vitis rOlumlilolia . [lot. Gaz. 96: 367-3 71.
1934·
5. ZlIDtER MAN, P. W ., and FRANK WILCOXON. Several chemic.-1.1growth substa nces which
cause lnitiation of roots and other responses in plants. Contrib. Boyce Thom pson
Inst.. 7: 2°9-229. 1935 ·
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