THE GROWTH o$ ROOTS AND STEMS. 187 



that help to support the stem, and therefore assist the plant 

 in climbing among the surrounding bushes and herbage. 



An acquaintance with plants in their habitats teaches 

 us that the formation and development of hairs, spines, 

 and prickles depend to a great extent on external conditions. 

 Thus, the same plant that produces these structures when 

 growing in a poor, dry soil fully exposed to the rays of the 

 sun, assumes a much softer and less aggressive character 

 when grown in a rich, moist soil. Under the former circum- 

 stances the plant, by converting some of its buds and leaves 

 into spines, reduces the amount of its foliage, and thus 

 economises its scanty supply of water. 



The Kest-harrow (Ononis arvensis) has no spines when 

 grown in moist soil, but in dry, exposed situations most of 

 the branches end in hard, sharp points. 



Many fleshy plants, however, which grow in dry situations, 

 such as the Stone-crop or the House-leek, show little or no 

 tendency to form hairs, prickles, or spines, since they have 

 other means of checking transpiration (thick cuticle, etc.). 



The hairs present on the stem and leaves are always cuti- 

 nised, and usually almost impermeable to water. They serve 

 to protect the plant, and especially the young growing organs, 

 from an excessive loss of water. Hairs, when thickly set, 

 also help to protect sensitive growing organs from too 

 strong light, which retards their growth and may injuriously 

 affect them. Similarly, a close covering of hairs is of some 

 importance in retaining heat during the night and thus 

 keeping the plant warm, while hairs are also of great value 

 in preventing the surface of the plant from being wetted by 

 rain. The hairs borne by the root (root-hairs), on the other 

 hand, have no cutin, and are very permeable to water. 



226. Boots as Storehouses of Food. In annual plants 

 the food produced by the plant during the growing season is 

 mostly turned at once into new tissues. The only store of 

 reserve food is that laid by in the seed for the use of the 

 young plant when germination occurs. In biennial plants 

 (which in the first year produce leaves and make and store 

 up food, and in the second produce flowers and fruits) reserve 

 food is often stored in the roots, which are greatly thickened, 

 as in Carrot, Beet, etc. In these cases the so-called tap-root 



