88 ELEMENTS OF PLANT ANATOMY. 
The leaf is in all cases undivided and, with the exception of 
the middle nerve, composed of one layer of cells. This middle 
nerve is lacking only in two genera, Fontinalis and Sphagnum. 
It consists of several layers and generally contains a bundle of 
long, thin-walled cells like those of. the central part of the stem. 
These bundles run back into the stem and anastomose with its 
byndle or bundles in a fanner quite similar to the so-called leaf- 
traces of higher plants. 
The form just described is known as the plant of the first 
generation. It produces an egg-cell or odsphere, which, after 
fertilization, develops into a plant of the second generation. 
This does not separate from the mother plant, and though 
abundantly supplied with chlorophyll, continues to derive its 
support in part from the mother plant. It is called a sporo- 
gonium like that in the hepatics and consists of stem and 
capsule. The only point of interest in this case is that not 
unfrequently stomata occur in the epidermis of this capsule. In 
this respect, therefore, the plant, so simple morphologically, is 
further advanced in anatomical differentiation than the moss 
plant with stem and leaves. 
3. Vascular Cryptogams (Pteridophytes). 
In this class alternation of generations is quite as sharply 
distinguishable as in the Bryophites, although the rank of the 
two generations is here completely reversed. We have seen in 
the Bryophites, the sexual or first generation represented by the 
conspicuous thallus or the leafy stemmed plant. This plant 
arises im all cases from an asexually produced spore whose 
method of germination resembles somewhat that of spores of 
lower forms. That is, it develops first into a more or less 
irregular growth from which the plant springs. This plant, 
whether it be thallus or stem, now develops exactly as all higher 
plants do—that is, from localized centres of growth and with 
more or less sharply differentiated tissues, 
