124 



THE CTYPTOGAMIA OB FLOWERLESS PLANTS. 



620. THE CAUSE OF THE DOWNWARD TENDENCY OF 

 THE ROOT is a theme of much discussion. Some have 

 referred it to the principle of gravitation j 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. 



CHAPTEK XV. 



THE CKYPTOGAMIA OB FLOWERLESS PLANTS. 



621. DISTINCTION OP PARTS. In the lowest 

 of the Cryptogainic tribes the organs of vege- 

 tation and of reproduction are the same. Each 

 cell in the structure grows, nourishes, multiplies. 



485, A Tree Fern (of the Island Highe ^ ^ SCale W6 find a gradual Specializa- 



of Java), 40 feet in height, tion ot organs, and in the higher tribes, as in 



A Fern, Polypodium 

 vulgare. 4S6, Its frond. 

 487, Lobe of the frond en- 

 larged, showing the sori, 

 4SS, One of the sori (mag- 

 nified) consisting of many 

 sporangi. 489, One spor- 

 ange (further magnified) 

 bursting and discharging 

 the spores. 490, A spore. 

 491, .Spores beginning to 

 germinate ; and 492, 493, 

 producing the prothallics 

 with rootlets. At a ap- 

 pear the antheridia and 

 at 5 the archegones on the 

 surface of the prothallus. 

 494, Antheridium. 495, 

 One of its cells. 496, The 

 same burst ; and 497, the 

 spermatozokJ escaped. 

 These float about, and 

 some of them at length 

 enter, 498, the archegone, 

 fertilizing, and at length 

 producing, 499, the young 

 Fern. 500, Sorns of As- 

 pidium marginale, covered 

 with the indusium. 501, 

 501 Same, side view. 



