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- 



