32 TEXTBOOK OF BOTANY 



slum. The flowers of the cobaea vine change from green to red, and 

 finally to violet, as they fade. Some rose flowers are pink when the buds 

 unfold and bluish when they fall. Flowers of the French hydrangea are 

 blue when the plants grow in acid soils containing salts of aluminum. 

 They are rose colored when the plants grow in alkaline limestone soils 

 where the aluminum salts are insoluble. It must be remembered that 

 the anthocyanins form a very large group of chemical compounds, and 

 the behavior of the pigments of any particular plant may be explained 

 only by the properties of the particular pigments present in that plant. 



Deciduous and evergreen habits. Another striking autumn phenomenon 

 is the falling of the leaves from many species of plants. This has been 

 shown to be definitely related to the shortening daily period of light. 

 During the summer a specialized layer of cells, called the absciss layer, 

 forms at the base of petioles and leaflets. The subsequent disintegration 

 of this layer may be started by conditions within the leaf brought about 

 by a variety of external conditions such as drought, change in length 

 of day, low temperatures, or leaf injuries. Hence any one, or all, of these 

 conditions may bring about leaf fall. Trees and shrubs that lose all or 

 nearly all of their leaves annually are said to be deciduous. 



In contrast to deciduous plants are those in which the life of any one 

 leaf extends through several years. These plants may have either broad 

 or needle leaves and their appearance varies but little from season to 

 season; the most familiar examples are the evergreen trees and shrubs. 

 Many herbaceous plants also have green leaves during the winter, as for 

 example, the common dandelion, evening primrose, teasel, and chick- 

 weed. These also might be classed as evergreens. 



Dormancy and periodicity. The gradual lowering of the temperature 

 and decrease in the length of the daily light period lead not only to 

 pigment changes and leaf fall, but to the death of many plants that 

 started from seed the preceding spring. Some part of the plant, how- 

 ever, remains alive and dormant throughout the late summer, autumn, 

 or winter. The part or parts that remain alive and doniiant vary greatly 

 with the kind of plant. If the plants have completed one generation or a 

 complete life cycle — including vegetative development, flowering, fruit- 

 ing, seed production, and death — within a single season, they are called 

 annuals. The dormant organ of such plants is the seed. The seeds of 

 some annuals, however, may also germinate in the autumn; the plants 

 pass the winter in the vegetative condition and bear seed the fol- 

 lowing spring. 



