1^4 



THE CTYPTOGAMIA OE FLOWBRLESS PLANTS. 



485 620. The cause of the downward tendency op 



THE ROOT is a theme of much discussion. Some hare 

 referred it to the principle of gravitation ; others to its 

 supposed aversion to light. But it ia a simple and satis- 

 factory explanation that its growth or cell-development 

 takes place most readily on the moist side of its growing 

 point, and consequently in a downward direction, so long 

 as the soil in contact with its lower surface is more moist 

 than that above. Hence also the well-known tendency 

 of roots toward springs and water-courses. 



CHAPTBE XV. 



(THE CETPTOGAMIA OB FLOWEKLESS PLANTS. 



621. Distinction of pabts. In the lowest 

 of the Cryptogamic tribes the organs of vege- 

 tation and of reproduction are the same. Each 

 cell in the structure grows, nourishes, multiplies. 



435, ATreeFer„(ofthe island ?g^"', '"^ *^« ''^^^ ^« ^""^ « gl-^dual specializa- 



ofjava), 40 feet in height, tiou ot Organs, and in the higher tribes, as in 



A Fern, Polypodinm 

 Tulgare. 4S6, Its frond. 

 4S7, Lobe of the frond en- 

 larged, showing the sori, 

 4SS, One of the sori (mag- 

 nified) consisting of many 

 sporangi. 489, One spor- 

 ange (farther magnified) 

 bursting and discharging 

 the spores. 490, A spore. 

 491, Spores beginning to 

 germinate ; and 492, 493, 

 producing the protkallus 

 ■with rootlets. At a ap- 

 pear the anth^ridia and 

 at 7> the archegones on the 

 surface of the prothallns. 

 494, Anthcridium. 496, 

 One of its cells. 496, The 

 same burst ; and 497, the 

 spermatozoid escaped. 

 These float about, and 

 some of them at length 

 enter, 498, the archegone, 

 fertilizing, and at length 

 prodnciDE. 499. the young 

 Pern. 500, Sorns of As- 

 pidiummarEinale, covered 

 ■with the indiMivm. 601, 

 Same, side view. 



