124 



THE CTYPTOGAMIA OK FLOWKRLESS PLANTS. 



620. THE CAUSE OP THE DOWNWARD TENDENCY OP 

 THE ROOT is 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 Cryptogamic 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- 

 tion of organs, and in the higher tribes, as in 



A Fern, Polypodium 

 vulgare. 4S6, Its frond. 



457, Lobe of the lioml en- 

 larged, showing the sori, 



458. One of the sori (mag- 

 nified) consisting of ninny 

 sporangi. 489, One spor- 

 ange (further magnified) 

 bursting and disehaiging 

 the spores. 490, A spore. 

 491, Spores beginning to 

 germinate ; and 492. 493, 

 producing the prothalln* 

 with rootlet?. At a ap- 

 pear the initfieHdia and 



^j c- -^^^r>: -^^ 



500 



at 6 thenrchegoni-s on the 

 surface of the piothallus. 

 494, Antheridium. 495, 

 One of its cells. 496. The 

 same burst ; and 497, the 

 gpermatozoid escaped. 

 These float about, anil 

 some of them at length 

 enter. 49S, the nrcheeom-. 

 fertilizing and at leneth 

 producing 4W. the yor.nir 

 Fern 500. Sonis of A*- 

 pidiiun mnnrinnlp. covered 

 with the indtiKivm. 501, 

 Same, side view. 



