. Class-book of botany [microform] : being outlines of the structure, physiology, and classification of plants : with a flora of the United States and Canada. Botany; Botany; Plants; Plants; Botanique; Botanique; Plantes; Botanique. 620. The cause of the downward tendency of THE ROOT i3 u theme of much discuswioii. Sotne have referred it to the principle of gravitation; others to its supposed aversion to liglit. 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 di
. Class-book of botany [microform] : being outlines of the structure, physiology, and classification of plants : with a flora of the United States and Canada. Botany; Botany; Plants; Plants; Botanique; Botanique; Plantes; Botanique. 620. The cause of the downward tendency of THE ROOT i3 u theme of much discuswioii. Sotne have referred it to the principle of gravitation; others to its supposed aversion to liglit. 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. CHAPTER XV. THE CRYPTOGAMIA OR FLOWERLESS PLANTS. 621. Distinction of parts. In the lowest of the Cryptogamic tribes the organs of v >- tation and of repioduction are the same. Each cell in the structure grows, nourishes, multiplies. ATn,eForn(of;hcisia„dH'^'^'"', '" *^" ^^^'^ ^« ^^^ a gradual specializa- ofJava), 40 feet in height, tion ot Organs, and m the higher tribes, as in Acin Acia Aaa 4.^7 Aoa 499 489. A Fern, rolypodiuni viilgnre. 4S6. Its frond. 487, Lobo of tlie frond en- larged, showing tlie sori. 4S8, One of the sori (ning- nifled) consisting df niiiny sporangi. 4S9, One spor- ange (further inngnified) bursting and disehnigiiiL' the spores. 490, A bporc. 491, Spores beginning to germinate; and 492. 40:i. producing llie protlmlbtx with rootlets. At <i Ap- pear tlio iivtheridia and at & the nreliegones on flic Bnrface of the prothallus. 494, Antiieridium. 495, One of its cells. 496. The same bur.«!t; and 497, the spennatozoid escaped. These float about, and somo of them at length enter. 49S, the archegone. fertilizinc, and at Ipnath prodncinsr. 499. the ymmg Fern .500. Poms of As- pidiiimmnririnalp. covered with the indvmvm. 601. Same, side Please note that
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