EVOLUTION AS SHOWN BY PLANTS 343 



In liverworts and mosses the sporophyte is small and 

 not green. Since it cannot make its own food it attaches 

 itself to the green gametophyte and lives upon it as a para- 

 site lives upon its host; yet it is no more an organic part of 

 the gametophyte generation than the mildew which grows 

 upon lilac leaves is a part of the lilac. 



In the next higher group of plants — which is the fern 

 group — the next great advance is to be observed. The 

 sporophyte has become green, can make its own food, and 

 is therefore independent of the gametophyte. Not only 

 that, but its green tissue has developed into leaves; it has 

 developed roots which connect it with the soil; and run- 

 ning from the roots up into the leaves there appears that 

 elaborate water-conducting system of tissue known as the 

 vascular system. This system is chiefly composed of the 

 woody strands so familiar as wood in the higher plants. 

 The ferns and their allies differ, therefore, from the liver- 

 worts and mosses in having independent leafy sporo- 

 phytes, with roots and a vascular system. 



With the large development of the sporophyte, and its 

 assumption of the task of food making with its large leaves, 

 and its continuation from season to season by means of 

 its fleshy underground stem, it seems natural to find the 

 gametophyte correspondingly reduced. In fact, although 

 leading an independent nutritive life, the gametophyte in 

 ferns is relatively inconspicuous. It looks like an ex- 

 ceedingly small liverwort; like a little green, heart-shaped 

 bit of leaf which often escapes attention entirely. It is 

 called the prothallium. Thus we see that the fern plant 

 of ordinary observation is a sporophyte or sexless plant, 



while the moss of ordinary observation is a gametophyte or 

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