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Laboratory 14 - Leptosporangiate Ferns: Reproductive Morphology
I.
Gametophyte and Gametangia
Obtain preserved specimens of the gametophytes of a typical fern. Notice the apical notch. Are there
antheridia and archegonia present? Observe the segmentation of the gametangia if they are present.
Obtain prepared slides of both archegoniate and antheridial gametangia. Although the ferns are
generally homosporous there is frequently a time delay in formation of either male or female
gametangia. What is the biological significance of this discoordinate development? First observe the
mature antheridia. Locate if possible, the opercular cell, first ring cell, and second ring cell. Can you
locate the primary spermatogenous cells or the sperms themselves? Where are the antheridia
positioned on the gametophyte? Locate the neck cells, neck canal cells and, if possible, the ventral
canal cell and the egg. What position do the archegonia occupy? Is this always a consistent feature?
Do filicalean gametophytes have an endophytic fungus? What are some other shapes which are
manifested in fern gametophytes?
II.
Sporangium
Observe living sporangia on a portion of a leaf of Polypodium. Notice the lack of an indusium
covering the sorus. Can you detect the single stalk and annulus of each sporangium? Have any of
the sporangia dehisced? If so, what is the direction and method of dehiscence? Obtain a prepared
slide of the sorus of Cyrtomium. Locate the base, stalk and umbrella of the indusium. What function
is served by the indusium? Locate the sporangium. Is the maturation of the sporangia simple,
gradate, or mixed? For each sporangium, locate the stalk, capsule, annulus, spores, and if possible,
the stomium. Is it possible to determine the developmental stages involved in the development of the
leptosporangium from this slide? How do leptosporangia differ from eusporangia in development
and mature structure? How is the tapetum organized within the leptosporangium? From what cell
initials is it derived? Can you locate the tapetum in the Cyrtomium sporangium? Locate a
sporangium showing a primary sporogenous cell and another showing the four or eight cell stages.
How many spores are ultimately produced by a typical leptosporangium?
III.
Soral Position and Associated Indusial Structures
Perhaps the most interesting facet in the study of the Filicales lies in the supposed trends of evolution
of soral position and the formation of the indusium. These trends are the subject of much controversy
and are entirely speculative, so be forewarned as you examine them. It does serve a purpose though
to illustrate morphological diversity within some framework, and that is why the concept of phyletic
slide is emphasized here.
Davalloid line
Within the typological series which terminates in the genus Davallia and related types there is a
progressive overtopping and outgrowth in margins. Consult the attached handout and phylogenetic
chart in Gifford and Foster to trace the typological progression. Observe the following:
Davallia - living material, prepared slide
Dicksonia sp. - prepared slide
Lygodium sp. - herbarium sheet, prepared slide*
Nephrolepsis sp. - living material, prepared slide
How has the concept of phyletic slide been utilized to explain this series?
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Pteroid Line
The line of supposed evolution leading from the marginal sporangia of Lygodium to the false
indusium of Pteris and Adiantum is said to result from the phyletic slide of the sporangium to the
adaxial surface of the pinna and a downward folding of the pinna margin. Trace this typological
series with the following examples:
Adiantum - slide, living material, herbarium material
Cheilanthes - herbarium specimen
Dicksonia - studied previously
Lygodium - studied previously
Pellea - prepared slide
Pteridium aquilinum - prepared slide*
Dryopterid Line
The most complex line of evolution within the ferns seems to be the Dryopterid line which has
seemingly given rise to number of diverse forms. The initial soral position is abaxial rather than
marginal and is best examined in the genus Gleichenia. The initial indusium appears cup-shaped and
appears to be derived from a scale-like indusium. Various types of soral and indusial types are also
derived including the vein-following coenosorus of Asplenium and the naked sorus of Polypodium.
Use the following examples:
Anthyrium - herbarium specimen
Asplenium - prepared slide
Cyathea - prepared slide*
Cyrtomium falcatum - prepared slide*
Cystopteris - prepared slide
Dryopteris - prepared slide
Gleichenia - prepared slide*
Polystichum - prepared slide
Woodsia - prepared slide
Gymnogrammoid Line
The Gymnogrammoid line is presumed to have begun from the primitive group related (or actually)
to Osmunda with marginal sporangia. A phyletic slide results in abaxial sporangia and a restriction
to the more marginal position yield the Gymnogramma or Cryptogramma type. Observe these:
Cryptogramma - prepared slides
Osmunda - prepared slides, herbarium sheets
Blechnoid and Onocleoid Line
Many members of this line of evolution are dimorphic in that certain whole fronds are fertile while
others are sterile. The protective indusia of the Blechnum-type is presumed to have arisen de novo
from initially reflexed leaf margins. Observe the following:
Gleichenia - observed previously
Onoclea sensibilis - prepared slide*, herbarium sheets
Hymenophylloid Line
As the name implies the Hymenophylloid ferns have thin, membrane-like leaves. In fact, the "filmyferns" are only one-leaf cell thick, usually with marginal, occasionally trumpet-like indusia.
Observe:
Hymenophyllum - prepared slide*, preserved material
Schizea - herbarium sheet, preserved material
Trichomanes crispum - prepared slides, preserved material