224: General Biology 



It will be seen from what has been said, that this whole group of 

 plants shows a differentiation of cells into tissues, while in the higher 

 forms leaf-like structures appear. Then the rhizoids (specialized absorb- 

 ing organs) are developed, and the plant tissues themselves contain 

 chlorophyl. It is supposed that bryophytes have evolved from aquatic 

 forms to land forms, and consequently, as parts of the plant have dried, 

 various structural adaptations ( ) have been brought 



about. 



PTERIDOPHYTES 



These are the ferns (Fig. 120) and their allies in which the dis- 

 tinguishing feature is that these plants possess nearly everything that 

 thallophytes and bryophytes possess, plus a conducting or vascular 

 system. 



These plants are supposed to have arisen "from a bryophyte ances- 

 try where the sporophyte (sexless) generation, in some plants capable 

 of doing chlorophyl work, developed a root system and vascular tissue, 

 and taking the land habit, became independent of the gametophyte. 

 This was one of the most important forward steps in the evolution of 

 the higher plants, for it gave the sporophyte complete freedom to live 

 and grow to its maximum size. This change marked a turning point 

 in plant evolution, for, after the sporophyte became the most complex 

 and conspicuous phase of the life-history, the gametophyte grew less 

 prominent, until in the seed-plants the sexual generation became actually 

 dependent or parasitic upon the asexual generation. This is a relation 

 which is exactly the reverse of that which exists between the gameto- 

 phyte and sporophyte in the liverwort and mosses." 



"After the sporophyte became independent of the gametophyte, the 

 next important advance was the development of the lateral spore-bear- 

 ing and vegetative organs called fronds ( ). Then 

 came the differentiation of the frond into vegetative leaves, given up 

 entirely to chlorophyl work, and spore-leaves (sporophyls) devoted 

 chiefly or wholly to spore production. With this also, came the massing 

 of the sporophyls into cones, which was really the beginning of the 

 structures called flowers in seed plants." 



The pteridophytes have underground stems (root-stocks or 

 rhizomes) so that only the leaves appear above ground. There is a 

 terminal bud at the tip of the fern-stem. The rhizome bears true roots, 

 and its tissues are differentiated into epidermal, fundamental, mechanical, 

 and conducting systems. In the tropics there are tree ferns, many of 

 which have been found among the fossil plants. 



The spore-cases grow in groups, called sori ( ), 



on the underside of the leaves (Fig. 121). As the annular ring about 



each individual spore-case dries up, that side which is thinnest and has 



become dried most, splits open, throwing out the spores. There are 



