38 TEXTBOOK OF BOTANY 



The development of branches and the enlargement of trunks of trees 

 and shrubs are greatly reduced. Soil water declines to the point where 

 many plants with shallow root systems wilt, and summer leaf fall may 

 take place. Some plants or plant parts may become dormant as a result 

 of high temperature and the long daylight period. 



There are fewer plants in bloom in midsummer than in spring or 

 autumn. The smaller grasses have attained maturity and completed their 

 life cycles. The larger grasses, such as corn, continue development and 

 bloom in summer. Summer is the period of greatest food accumulation 

 in the stems and roots of biennials and perennials. It is also the period 

 when fungi and bacteria cause numerous plant diseases and rapid decay 

 of organic matter. Insects also have reached maximal abundance, and 

 their injuries to leaves and stems and fruits become most apparent. 



Some plants bloom onlv when the days are long and the nights are 

 short. As a result the summer season is the time of flowering of corn, 

 clover, mallows, cotton, and many other plants. 



As the days become shorter at the close of summer and the tempera- 

 tures decline, increased water is available in many parts of the United 

 States because less of the rainfall is lost by evaporation. The water rela- 

 tions of plants are improved over those of the summer and the growth 

 of many herbaceous plants increases. Not infrequently a second wave of 

 flowering occurs in some plants, such as violets, that bloom abundantly 

 during the spring months. 



But the further decline in temperature brings the conditions of au- 

 tumn, and we have completed our very brief view of the more noticeable 

 seasonal phenomena. We have observed that certain vegetational aspects 

 are rather characteristic of each season; that some phases of growth 

 belong to one season rather than to another; and that dormancy, growth, 

 maturity, and death of plants form a regularly recurring cycle. 



The problems of plant science are all about us and each season 

 brings its own challenges to investigation and understanding. We need 

 confine our outdoor study of botany neither to a single environment, to 

 but one season of the year, nor to any one locality. 



REFERENCES 



(Chiefly on plant pigments) 

 Kuhn, R. Plant pigments. An/i. Rev. of Biochem. 4:479-496. 1935. 

 Mobius, M. Pigmentation in plants, exclusive of the algae. Bot. Rev. 3:351- 

 363. 1937. 



