‘ 
1878. | On the Genealogy of Plants. 363 
exhibit a species of exogenous growth. This differs not only 
from that of the existing pines and from that of the true Exo- 
gens, but also from that now known to take place in certain 
monocotyledonous plants and constitutes a sort of fourth type. 
It consists, so far as understood, in the formation of a layer of 
growing tissue with dividing cells (meristem) around each fibro-" 
vascular bundle, the continuous division of whose cells necessi- 
tates a radial or centrifugal increase of the entire stem. A simi- 
lar structure on a small scale occurs in certain now living crypto- 
gamic forms, as in Botrychium, Isoëtes, etc. This form of exog- 
enous growth may perhaps be regarded as marking a transition 
from the endogenous structure of most cryptogamic stems to the 
form of exogenous structure which prevails in the Conifere, but 
which has not been transmitted to the branch from which, on the 
hypothesis of descent, the Cycadacee have been developed. 
The transition in the reproductive system is far more obvious 
and remarkable. What is known as “alternate generation,” so . 
long familiar to zodlogists, is now found to prevail throughout 
the greater part of the vegetable kingdom. It is most apparent 
in the higher Cryptogams, especially in the mosses, ferns, Ægui- 
setacee and Lycopodiacee. In all these the final stage is the pro- 
duction of a plant or “ generation” capable of déveloping spores, 
which are of both sexes, and produce the sexual plant. Among 
the vascular Cryptogams there are two orders, one the Rhizocar- 
pee@ in the fern group, the other the ZLzgu/ate in the club-moss 
group, in which the final spore-bearing stage is sexually differ- 
entiated. These produce two kinds of spores, called respectively, 
from their relative size, sacrospores and microspores, the former 
of which develops a female, and the latter a male prothallium, or 
sexual plant. This protha/lium, which in most vascular Crypto- 
gams is an object of considerable size, and which corresponds to 
the entire leafy portion of the mosses, liverworts and other cellu- 
lar forms, continues to diminish as the degree of organization 
increases; the spore-bearing generation, on the contrary, increas- 
ing ina omero ratio; the large fronds of a fern repre- 
senting only the seta and capsule, or fruiting portion of a moss. 
n the Rhizocarpee and Ligulate, whose macrospores and micro- 
spores indicate a higher organization, this reduction of the sexual 
generation is carried so far that the prothallium scarcely protrudes 
from the spore, or is wholly confined within it. Hoffmeister’ has _ 
1 Vergleichende Untersuchungen, 1851. 
