168 



BOTANY 



PAKT I 



as the RADICLE. The tap-root of the fully-developed plant is formed 

 by the prolongation of the radicle and later gives rise to lateral roots. 

 The figure shows the germination at successive stages from the 

 longitudinal section of a mature seed (A) to the seedling with 

 unfolded cotyledons (E). In Fig. 173 a germinating plantlet of the 

 Hornbeam (Carpinus Be(ulus) is shown with its hypocotyl (h) and 

 two cotyledons (c) ; its radicle has already developed into a tap-root 

 (hiv) with a number of lateral roots (siv). An internode and foliage 



leaf (/) have been produced 

 from the vegetative cone of 

 the stem ; while the next 

 higher internode is also dis- 

 tinguishable, but has not 

 yet elongated, and a second 

 foliage leaf (/') is unfolding. 

 A highly organised 

 plant, Avhich begins its 

 development with the 

 simplest stages and gradu- 

 ally advances to a higher 

 state of differentiation, to 

 a certain extent repeats 

 in its ontogeny its phylo- 

 genetic development. In 

 the process of its onto- 

 genetic development much 

 has been altered, and much 

 omitted, so that it presents 

 but an imperfect picture of 



Fig. 174. — Seedling of Acacia yycnantha. The cotyledons jj^g i-)og^ historv ' nCVCrtlie- 



liavc been thrown off. The foliage leaves ] -4 are pinnate, •, , . j. j. • 



the following leaves bipinnate. The petioles of leaves '^^^) ^^^^ representation IS 



5 and 6 are vertically expanded; and in the following- Valuable, and, UCXt tO COm- 



leaves 7, 8 9, modified as phyllodes, bearing nectaries, p^rative methods, fumisllCS 



n. (About ^ natural size.) ^ . ' 



the most important source 

 of our morphological knowledge. Whatever is true of the develop- 

 ment of a plant from the embryo is also, as a rule, applicable 

 to its further growth from the growing point, and, consequently, 

 a knowledge of the mode of development at the grooving point 

 is of great importance in establishing 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 between the 

 different plants ; the later it is exhibited in the embryo, or the 

 farther removed it is from the growing point of the plant, the less 

 its general value, but the greater, in proportion, its importance in 

 defining the character of a genus or species. From the fossil remains 



4 



