46 



BOTANY 



PAKT I 



COTYL, the first leaves (c) are the seed leaves or cotyledons, while 

 the root (r) is distinguished as the radicle. The tap-root of the fully- 

 developed plant is formed by the prolongation of the radicle. In 

 Fig. 47 a germinating plantlet of the Hornbeam (Carpinus Betulus) is 

 shown with its hypocotyl (h) and both cotyledons (c) ; but its radicle 

 has already developed into a tap-root (hw) with a number of lateral 

 roots (sw). An internode and foliage leaf (I) have been produced 

 from the vegetative cone of the stem ; while the next higher internode 

 is also distinguishable, but has not yet elongated, and a second foliage 

 leaf (V) is unfolding. 



A highly organised plant, which begins its development with 

 the simplest stages and gradually advances to a higher state of 

 differentiation, repeats in its ontogeny its phylogenetic develop- 

 ment. In the process of its ontogenetic development much has 

 been altered, and much omitted, so that it presents but an imperfect 

 picture of its past history ; nevertheless, this representation is valuable, 

 and, next to comparative methods, furnishes the most important source 

 of morphological knowledge. Whatever is true of the development of 



a plant from the embryo 



is also, as a rule, applicable 

 to its further growth from 

 the growing point, and, con- 

 sequently, a knowledge of 

 the mode of development 

 at the growing point is 

 of great importance in 

 detecting homologies. The 

 earlier a characteristic 

 makes itself apparent in 

 the embryo, or the nearer 

 it is to the growing point 

 of the old plant, so much 

 the greater is its value in 

 determining the general 

 relationships existing be- 

 tween the different plants ; 

 the later it is exhibited in 

 the embryo, or the farther 

 removed it is from the 



-Seedling of Acacia pycnantha. The cotyledons growing point of the T)lant 



have been thrown off. The foliage leaves 1-4 are li i ., l i ' 



pinnate, the following leaves bipinnate. The petioles e Iess lts general Value, 



of leaves 5 and 6 are vertically expanded ; and in the but the greater, in propor- 



» modified as phyiiodia, with t i on> itg i mpor tance in de- 

 fining the character of a 

 genus or species. From the fossil remains of former geological periods 

 it is safe to conclude that such Conifers as Thuja, Biota, and the various 



Fia. 48.- 



following leaves, 7, 8, 9, 

 nectaries, n. ( x circa £.) 



