Part 12 (1/2)

All of the organs of the fern grow from a definite apical cell, but it is difficult to study except in the root

Selecting a fresh, pretty large root, a series of thin longitudinal sections should be ers or placing it between pieces of pith In order to avoid drying of the sections, as is indeed true in cutting any delicate tissue, it is a good plan to wet the blade of the razor If the section has passed through the apex, it will show the structure shown in Figure 68, _D_ The apical cell (_a_) is large and distinct, irregularly triangular in outline It is really a triangular pyramid (tetrahedron) with the base upward, which is shown by h the root tip, and coitudinal sections The cross-section of the apical cell (Fig _L_) appears also triangular, showing all its faces to be triangles Regular series of segments are cut off in succession froular divisions also, so that very early a differentiation of the tissues is evident, and the three tissue systeround, and fibro-vascular) may be traced almost to the apex of the root (68, _D_) Fro the delicate growing point and protecting it fro otherwise protected, develop segments only fro segments cut off from it

CHAPTER XIII

CLassIFICATION OF THE PTERIDOPHYTES

There are three well-marked classes of the Pteridophytes: the ferns (_Filicinae_); horse-tails (_Equisetinae_); and the club mosses (_Lycopodinae_)

CLass I--FERNS (_Filicinae_)

The ferns constitute by far the greater nueneral structure corresponds with that of the maiden-hair fern described There are three orders, of which two, the true ferns (_Filices_) and the adder-tongues (_Ophioglossaceae_), are represented in the United States A third order, intermediate in soless ferns (_Marattiaceae_), has no representatives within our territory

The classification is at present based largely upon the characters of the sporophyte, the sexual plants being still very iues (_Ophioglossaceae_) arefroenera in the United States, the true adder-tongues (_Ophioglossurape ferns (_Botrychiu speci 70, _A_) is divided into two portions, the spore bearing (_x_) and the green vegetative part In _Botrychiuia distinct (Fig 71, _B_) In _Ophioglossum_ the sterile division of the leaf is usually s division foria are ia in both differ essentially frole epiderround tissue of the leaf

[Illustration: FIG 70--Forrape fern (_Botrychiuia of _Botrychiu leaflets,_D_, a sporangiu _E_, _Polypodium_, 1 _F_, brake (_Pteris_), 1 _G_, shi+eld fern (_Aspidium_), 2 _H_, spleen-wort (_Asplenium_), 2 _I_, ostrich fern (_Onoclea_), 1 _J_, the saes of the leaflet partially raised so as to show the ia beneath, 2]

In the true ferns (_Filices_), the sporangia rese in all (unless possibly _Osroup, the water ferns (_Rhizocarpeae_), produce two kinds of spores, large and small The former produce male, the latter female prothallia In both cases the prothallium is small, and often scarcely protrudes beyond the spore, and71, _B_, _C_) with only one or two cells representing the vegetative cells of the prothallium (_v_) The water ferns are all aquatic or semi-aquatic plants, few in number and scarce or local in their distribution The co 71, _A_), looking like a four-leaved clover Others (_Salvinia_, _Azolla_) are floating for 71, _D_)

[Illustration: FIG 71--_A_, _Marsilia_, one of the _Rhizocarpeae_ (after Underwood) _sp_ the ”fruits” containing the sporangia _B_, a small spore of _Pilularia_, with the ripe antheridiu, 180 _C_, male prothalliuue), 1]

Of the true ferns there are a nuuished ia, as well as by some differences in their structure Of our coferns (_Osia are large and the ring (_r_) rudiia are more or less contracted and covered coia, so thus changed, soured

Our other co

_sorus_) on the backs of the leaves These sori are of different shape in different genera, and are usually protected by a delicate(indusiuenera are shown in Figure 70, _E_, _J_

CLass II--HORSE-TAILS (_Equisetinae_)

The second class of the pteridophytes includes the horse-tails (_Equisetinae_) of which all living forenus (_Equisetum_) Formerly they were much more nu especially abundant in the coal for stem of the field horse-tail (_Equisetu cone _B_, sterile steround stem, with tubers (_o_),_D_, cross-section of an aerial stele fibro-vascular bundle, 150

_tr_ vessels _F_, a single leaf froh a spore sac (_sp_), 5 _H_, a spore, 50

_I_, the same, moistened so that the elaters are coiled up, 150

_J_, a male prothallium, 50 _an_ an antheridium _K_, spermatozoids, 300]

One of the commonest forms is the field horse-tail (_Equisetum arvense_), a very abundant and widely distributed species It grows in low,in the sand or gravel used as ”ballast” for railway tracks