ENVIRONMENT AND HABIT 119 



Again in countries which have two well marked 

 seasons to the year, a warm moist period when plants 

 can grow actively (as after the rains in India, and 

 during the early summer months in temperate climates),, 

 followed by a very hot and dry, or a very cold season, 

 when all tender herbaceous vegetation is killed off,, 

 it is of advantage to a plant if it possesses some 

 part that will live on through this latter period, and 

 from which new shoots can spring up as soon as ever 

 the climate allows, thus taking an early advantage of 

 the favourable season. 



The need of this perennial part is supplied by the 

 bulb, corm, tuber or root-stock as the case may be. 

 It always contains a large quantity of water to enable 

 it to live on during the season, when the roots cannot 

 supply moisture because of the dryness or the cold, 

 and also a certain amount of carbonaceous food- 

 material, packed away generally in the form of starch 

 as in the potato and other stem- or root-tubers, but 

 sometimes partly as sugar, e.g. in the onion-bulb. 

 These carbohydrates are used up at the beginning 

 of the growing season to make the new shoot, and if 

 a young potato plant be dug up, the tuber from 

 which it grew will be found an empty and shrivelled 

 skin, or a mere slimy mass much less than its. 

 original size. 



During the vegetative season the first work of 

 the plant, after making its stem and leaves, is to 

 provide for the seeds. In them nitrogenous and car- 

 bonaceous food-material is packed away in even more 

 concentrated form and with scarcely any water, and 

 it is only after the year's seeds have been provided 



