124 



THE CTYPTOGAMIA OR FLOWKELESS PLANTS. 



485 



620. THE CAUSE OF 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- 



4S5 A Tree Fern (of the Island . - , . ,, , . & . 



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



A Fern, Polypodium 

 vulgare. 4S6, Its frond. 

 4S7, Lobe of the frond en- 

 larged, showing the sori, 

 4SS, One of the sori (mag- 

 nified) consisting of ninny 

 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 prothallus 

 with rootlets. At a ap- 

 pear the untheridia and 

 at & thenrchegones on the 

 surface of the prothallns. 

 494, Antheridium. 495, 

 One of its cells. 496, The 

 same burst ; and 497, tho 

 spennatozoid escaped. 

 These float about, and 

 some of them at length 

 enter, 498, the archegone, 

 fertilizing, and at length 

 producing, 499, the young 

 Fern. 500, Sorus of As- 

 pidiummarginale, covered 

 with the indusitan. 501, 

 Same, side view. 



492 



493 



