In the lower plants the starch stored during the day is utilized during darkness, mainly for tissue respiration. In higher forms, however, it serves two main purposes in addition to ’tiding over ’the organism through the winter when it is inactive : that of abnormally rapid growth when activity is resumed ; and that of vegetative reproduction, I.e. that due entirely to growth. With the exception of the evergreen plants, on the approach of spring, no growth could occur in, I.e. those that complete their life history in two years, or in perennials, since they possess no photosyntheiic organs for making food at that time. Stored food is therefore mobilized and transported to actively growing regions where the cells change from tiny ones tightly packed together to large highly vacuolated ones. This is readily seen in spring by standing horse chestnut twigs in water in a warm place, when the buds will break and will develop into quite large shoots. The development of a bud really occurs in two stages : firstly, the unfolding of the bud with the attendant elongation of the internodes, due largely to vacuolation of the cells by water intake, and secondly, the true growth as die make food and other food is transported into the shoot from elsewhere. The twig should have its end cut under water to prevent the entry of any air into the water-conducting vessels of the xylem, thereby enabling water to be most easily absorbed.
A moment ’s thought will make clear the fact that those plants which appear first and flower early in die year grow up from special storage organs. Many arein contrast to the true perennials. The biennials are all , and therefore can be grown from . The first year at least is normally spent in producing leaves, the food made by them being stored in the storage organ underground. With the approach of winter the portions above ground die off, to be replaced by new leaves and the following spring, accompanied by the production of followed by fruit, completing the life cycle. Sometimes, if conditions during the first year are unfavourable, the plant attempts to complete its life history in one year and ’runs to seed, ’ or ’bolts, ’ to use the gardener ’s phrase, a state commonly exhibited amongst lettuces and beetroot.