PTERIDOPHYTES 325 
to appear very early, and later the archegonia. If the pro- 
thallium is poorly nourished, only antheridia appear; it 
needs to be well developed and nourished to develop arche- 
gonia. There seems to be a very definite relation, there- 
fore, between nutrition and the development of the two sex 
organs, a fact which must be remembered in connection 
with certain later developments. 
213. The sporophyte, This complex body is differ- 
entiated into root, stem, and leaf, and is more highly 
organized than any plant body heretofore mentioned (Fig. 
296). 
In most of the Ferns the stem is subterranean and dor- 
si ventral (Fig. 296), but in the " tree ferns " of the tropics 
it forms an erect, aerial shaft bearing a crown of leaves 
(Fig. 297). In the other groups of Pteridophytes there are 
also aerial stems, both erect and prostrate. The stem is 
complex in structure, the cells being organized into differ- 
ent " tissue systems," prominent among which is the vascu- 
lar system- 
One of the peculiarities of ordinary fern leaves is that 
the vein system of the leaves branches dichotomously, the 
forking veins being very conspicuous (Fig. 298). Another 
fern habit is that the leaves in expanding seem to unroll 
from the base, as though they had been rolled from the 
apex downward, the apex being in the centre of the roll 
(Fig. 296). This habit is spoken of as circinate, from a 
word meaning " circle " or " coil," and circinate leaves when 
unrolling have a crozier-like tip. The arrangement of 
leaves in bud is called vernation (" spring condition "), and 
therefore the Ferns are said to have circinate vernation. 
The combination of dichotomous venation and circinate 
vernation is very characteristic of Ferns. 
214. Sporangia. The sporangia are borne by the leaves, 
generally upon the under surface, and are usually closely 
associated with the veins, and organized into groups of defi- 
nite form known as sori. A sorus may be round or elon- 
