no TA NIC A L TKll MS. 



45 



I 



Stinghifj hairs secrete a poisonous liquid. Tlio point 

 of a hair of tliis kiiul on piercing; the skin breaks ofT, 

 leaving the poison in the wounJ. 



Prickles (Fig. 249) differ from thorns in being out- 

 growths of the bark • thorns arise from the wood. 



GROWTH. 



The growth of a plant consists in llio multiplication of its cells, 

 and the subsequent enlargement of the latter by the addi- 

 tion of new matter. The development of the cell frequently 

 Involves, also, a change of form. 



Germination. 



By this term is meant the commencement of the process of 

 growth from the seed. Under suitable conditions of 

 tempeiature and moistui-e the embryo, which is dormant 

 in the dry seed, wakens into activity and begins to 

 develope. The details of the process vary somewhat 

 according to the structure of the seed. If the cotyledons 

 are thin and leaf-like, as in Majtle for example, the radi- 

 cle genei-ally grows througliout its length so as to raise 

 them above the soil, wliere they at once expand and 

 become the first green leaves of the new plant, a root 

 being at the same time developed from the lower end of 

 the radicle. But if the cotyledons are thick and llosliy, 

 containing much nourishment, then usually a bud called 

 the plumule, which contains the elements of additional 

 bits of stem, will be a prominent feature in the embryo, 

 and in this case the cotyledon or cotyledons not infre- 

 quently remain under ground, as in the pea and the 

 acorn, and so do not perform the office of foliage-leaves, 

 but merely supply the newly developing parts with nour- 

 ishment. In albuminous seeds, the endosiierm is the 

 chief source from which the germirating embryo derives 

 its support. 



Vitality of Seeds. 



There is a considerable difference in regard to the length of 

 time during which seeds retain their vitality. Some, 

 such as those of Elm and Poplar, will germinate only 

 if they have been kept fresh and not permitted to dry 

 up, while others, such as those of Indian Corn and , 

 Wheat, and in general those containing a copious store of j 

 starch, may be kept for a very long time without lo-sing | 

 their germinating power. 



