﻿Trees of New York Si ate 17 



second group, the liverworts, is also included. The Bryophytes 

 show a distinct advance in specialization over the Thallophytes. 

 This is evinced through the definite establishment of a sexual 

 stage in which the sexes may be distinguished, and an "alternation 

 oC generations" whereby a sexual stage or generation is followed 

 by a semi-dependent, asexual stage which in turn again gives 

 rise to sexual forms. While more specialized than Thallophytes, 

 Bryophytes are, relatively speaking, simple plants. The plant 

 body is an elementary structure which possesses chlorophyll and is 

 in some cases thalloid, while in others it develops a primitive stem 

 and leaves. True vascular tissue (vascular bundles) is entirely 

 lacking. 



Vascular plants make their appearance for the first time in the 

 Pteridophytes, a group which includes the true ferns and what 

 are recognized as fern allies, the horsetails, scouring rushes, 

 club mosses, and quillworts. True roots, stems, and leaves 

 equipped with special conducting or vascular tissue, have become 

 established as definite structures and function as in the seed 

 plants. As in the Bryophytes there is a sexual stage in which 

 the sexes may be distinguished but the sexual organs have be- 

 come increasingly specialized. This is followed by an asexual 

 stage in which sexless individuals through spore formation again 

 give rise to sexual forms. In the higher Pteridophytes it is the 

 asexual or sporophytic stage that has become dominant while 

 the sexual generation has been relegated to an obscure, indepen- 

 dent existence or has become actually parasitic on the asexual 

 generation. Pteridophytes were formerly represented by a vast 

 assemblage of plants many of which were arborescent and 

 flourished during the Carboniferous period, contributing largely 

 in the formation of our coal deposits of today. Owing to an 

 altered environment and the development of seed plants which are 

 better adjusted to withstand modern conditions the group is now 

 on the wane and is represented only by some 4000 species. 



The dominant plants of today are the seed plants or Sper- 

 matophj'tes. They represent the highest type of specialization, 

 though not necessarily the final type. Like the Pteridophytes 

 they bear true roots, stems, and leaves and have an independ- 

 ent asexual or sporophytic stage on which the sexual or gameto- 

 phytic stage is wholly dependent. The most striking difference 

 lies in the formation of seeds which are dormant structures rep- 

 resenting a pause in the development of the new sporophyte, de- 

 signed by nature to tide the plant over unfavorable periods and to 



