PTERID O PHYTA 
549 
whilst the sporogonium of these latter is the equivalent of the * plant * 
in a fern or lycopod. 
The plant itself takes various forms in the different groups. Except 
in the tree ferns and in fossil forms it does not attain any great size. 
There is an erect stem in many ferns &c. ; others have creeping stems 
(e.g. Lycopodium and Selaginella), rhizomes (many Ferns), or floating 
stems (Hydropterideae). The leaves are simple, except in many ferns. 
There is no primary tap-root, but roots are formed as required from 
the stem or leaves. Internally there are well marked vascular bundles 
in both stem and leaf, and many of the anatomical features of Phanero- 
gams may be found here also. 
The sporangia arise upon the leaves, either on the ordinary foliage 
as in most ferns, or on specially differentiated leaves as in Osmund a, 
Equisetum, Lycopodineae, Hydropterideae, &c. They may be solitary 
or in groups (sort) ; in the latter case they are often protected by a 
special outgrowth of the leaf, the indnsium. The spores are formed 
by a complicated process from a single cell or row or layer of cells — 
the archesporium — inside the sporangium : each has a thick waterproof 
outside wall. The spores may be of one kind only, in which case the 
plant is termed homo - or iso-sporous, or of two kinds ( heterosporous ). 
In this case the smaller spore is termed the microspore , the larger the 
mega- or macro-spore \ the former gives rise to a male prothallus, the 
latter to a female. 
Polling upon the soil (or into the water, in the case of the 
Hydropterideae) the spores germinate under suitable conditions, 
giving rise to the sexual plants or prothalli . The prothallus is a 
small body without distinction into stem and leaf; it absorbs materials 
from the soil (usually by rhizoids) and, being green, assimilates in the 
ordinary way. It bears the sexual organs — antheridia (male) and 
archegonia (female). In the homosporous forms these are both found 
on the same prothallus, except in Equisetum, where, though the 
spores are absolutely similar so far as we can tell, there are separate 
male and female prothalli, as in the heterosporous forms. In the 
antheridia are developed the motile male cells or antherozoids (often 
called spermatozoids as they are equivalent to the spermatozoa of 
animals). Fertilisation takes place by aid of water. The mucilage 
contained in the neck of the archegonium is attractive to the sperma- 
tozoids (p. 56), which swim up the neck of the archegonium. One 
of them finally fuses with the ovum or female cell at the base of the 
archegonium, and the fertilised ovum (zygote) then developes into a 
new ‘plant’ or asexual generation, being nourished by the prothallus 
until it can assimilate for itself. 
Classification : 
I. Filicineae : Mostly homosporous, with monoecious prothalli 
(Hydropterideae are heterosporous, with dioecious prothalli). 
Stem with few or no branches ; leaves usually large and 
