162 



BOTANY 



The stage of the ontogeny of the plant known as GERMINATION 

 is specially defined in Phanerogams, where the term is limited to 

 the further growth of the embryo already present within the 

 seed. The embryo, enclosed within the seed, by the time the latter 

 is shed from the parent plant generally exhibits the segmentation 

 characteristic of Cormophytes. Protected by the hard seed-coats, it 

 is enabled to sustain a long period of rest. Abundant deposits of 



nutritive material in the 

 embryo itself, or surround- 

 ing it, are provided for its 

 nourishment during ger- 

 mination. The different 

 segments of a phanerogamic 

 embryo have received dis- 

 tinctive names ; thus, as in 

 the embryo of the American 

 Arbor Vitae (Thuja occiden- 

 talis, Fig. 169), the stem 

 portion (h) is termed the 

 HYPOCOTYL, 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 prolonga- 

 tion of the radicle and later 

 gives rise to lateral roots. 



FIG. 171. Seedling of Acacia jiycnantha. The cotyledons J n ~p j 170 a germinating 



have been thrown off. The foliage leaves 1-4 are . P" . ** _ , 



pinnate, the following leaves bipinnate. The petioles plantlet Ol the Hornbeam 



of leaves 5 and 6 are vertically expanded ; and in the (CdTpinUS jBetulus) is shown 



following leaves ^8 9, modified a.s phyllodia, bearing j fch j fc hypocotyl /) an( J 



nectaries, n. (About J natural size.) J r J \ / 



two cotyledons (c) ; 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 (I 1 ) is unfolding. 



A highly organised plant, which begins its development with the 

 simplest stages and gradually advances to a higher state of differentia- 

 tion, to a certain extent repeats in its ontogeny its phylogenetic 

 development. 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 our morphological knowledge. Whatever is true 



