[Chap. IV SEASONAL ASPECTS OF PLANTS 37 



which buds on different plants open first and from what buds the current 

 year's branches develop, whether the extension of the main stem or 

 branch always develops from the terminal bud, and if the branches from 

 all lateral buds grow equally. You can also ascertain whether leaves and 

 flowers develop on twigs from the same or from separate buds, which 

 of our common trees blossom before the development of leaves, and 

 whether these flowers are borne on twigs of the previous year or only on 

 new twigs. These are but a few of the questions that may be answered 

 by a study of woody plants in the field. 



With the coming of spring countless millions of seedlings appear in 

 every unoccupied plot of ground. New branches develop on perennials 

 and biennials, and leaves are followed soon after by masses of flowers of 

 every imaginable hue. Spring is the period of most rapid development 

 in most plants, partly because the overwintering parts have been "condi- 

 tioned" by the low temperature and partly because of increased light, 

 increased length of day, and an abundance of available water in the soil. 

 After growth has started it may be retarded either by drought or by low 

 temperatures. On the other hand, elongation of the stems and expansion 

 of the leaves of most trees and shrubs stop within a few weeks, even if 

 temperature and moisture conditions continue unchanged. The growth 

 of new stem segments is definitely limited by internal conditions and 

 their elongation usually ceases by late May or June in northern latitudes. 

 Sprouts from stumps and pollarded trees continue to elongate for several 

 weeks more. 



This is the period when many of the flowering plants of densely 

 shaded woods have their annual development, flower, fruit and return 

 to dormancy. In the open there are many plants in which the period of 

 development continues into the summer and autumn before flower, fruit, 

 and seed fomiation closes the annual cycle. 



Spring, then, is the period of most active growth of roots and shoots 

 of woody plants. It is the period of most active utilization of food, and 

 the period of rapid respiration and food manufacture. The high rates of 

 these processes stand in sharp contrast to their much lower rates during 

 the winter season. 



Summer aspect. With the coming of the longest days temperatures 

 also are high, and available soil water on the average begins to decline. 

 Growth of plants as a whole also declines, but many of the flowers of 

 springtime are now being followed by fruits and seeds. 



