124 



THE CTYPTOGAMIA OB FLOWBRLESS PLANTS. 



4S5 



620. The cause of the downward tendency of 

 the ROOT i3 a theme of much discussion. Some have 

 referred it to the principle of gravitation ; others to its 

 supposed aversion to light. But it is 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. 



CHAPTER XV. 



THE CRYPTOGAMIA OR FLOWERLESS PLANTS. 



621. Distinction of parts. In the lowest 

 of the Cryptogamie tribes the organs of vege- 

 tation and of reproduction are the same. Each 

 cell in the structure grows, nourishes, multiplies. 

 Higher in the scale we find a gradual specializa- 

 ^iv^t ( fnt e Sr d tion of organs, and in the higher tribes, as in 



A Fern, Folypodinm 

 vnlgare. 4S6, Its frond. 

 4S7, Lobe of the frond en- 

 larged, showing the sori, 

 4SS, One of the sori (mag- 

 nified) consisting of ninny 

 sporangi. 4S0, One spor- 

 ange (further magnified) 

 bursting and discharging 

 thespoies. 490, A spore. 

 491, Spores beginning to 

 germinate; and 492, 493, 

 producing the prothallv* 

 with rootlets. At a ap- 

 pear the tmltiericHa and 

 at b the nrehegones on the 

 surface of the prothallus. 

 494, Antheridium. 496, 

 One of its cells. 496, The 

 same burst ; and 497, tho 

 sperm atozoid escaped. 

 These float about, and 

 some of them at length 

 enter. 49S, the arehegone, 

 fertilizing, and at length 

 producing 499. the young 

 Fern. 500, Sorns of As- 

 pidinmmarginalc, covered 

 with the indvsium. 601, 

 Same, side view. 



