568 GROWTH, ASSIIVIILATION, AND ACCUMULATION 



shape representing the grand period of growth. During the earlier stages of 

 this period the leaf area of the plant and hence its photosynthetic capacity 

 increase at an accelerated rate. Correlated with this is a progressive accelera- 

 tion in the rate of increase in dry weight. Some of the increase in dry weight 

 is also due to the absorption of ions from the soil but proportionately this is 

 always very small. Midway of the grand period a reversal in trend becomes 

 apparent. From this point on there is a gradual deceleration in growth incre- 

 ment until increase in dry weight ceases entirely. As the plants mature the 

 photosynthetic efficiency of the leaves decreases, and in some species leaves may 

 actually be shed during this period. Reproductive development usually occurs 

 during this period of deceleration in rate of increase in dry weight. Most of 

 the foods synthesized are diverted into the developing fruits and seeds, con- 

 sequently vegetative growth is suppressed and new leaves are not produced 

 with sufficient rapidity to compensate for the loss of photosynthetic activity in 

 the older leaves. 



Finally the plant passes into a state of senescence during which the respira- 

 tion rate usually exceeds the rate of photosynthesis. Hence during this period 

 of decline the plant loses dry weight. The later stages in the ripening of 

 fruits and seeds often occur during this period of senescence ; in fact the high 

 respiratory activity of such organs often accounts for much of the loss of dry 

 weight during this period. Abscission of leaves and fruits may also be re- 

 sponsible for a considerable proportion of the loss in dry weight during 

 senescence in some species. 



The growth cycle of an annual plant is terminated by its death at which 

 time it still usually retains a large proportion of the substance produced as 

 a result of its synthetic activity. Much of this is in the form of the cellular 

 structure of the plant body, but in many plants a considerable proportion of 

 the accumulated dry matter represents assimilated and accumulated foods in 

 the ripened fruits and seeds, some or all of which are retained in many annual 

 species until after the death of the vegetative organs. 



Rates of Growth, — The absolute growth rates recorded in the botanical 

 literature are mostly expressed in terms of increase of height of stems, although 

 some figures are available in terms of dry weight increment and other units. 



The rate of height growth varies enormously with different species of 

 plants, and with the same species under different environmental conditions. 

 Only a few examples of the most rapid known rates of height growth will be 

 cited. Young bamboo shoots occasionally grow as rapidly as two feet in 

 twenty-four hours. When a flowering stalk is produced by the century plant 

 it often elongates as much as six inches during the course of a single day. 



