ACCUMULATION OF FOODS 569 



Under favorable growing conditions corn plants sometimes add visibly to 

 their stature during a single night. 



The rates of elongation of the fastest growing stems are just a shade too 

 slow for detection with the naked e_ve. By observing rapidly growing stem 

 tips under a horizontally placed microscope the externally visible aspects of 

 growth can often be observed and measured directly. 



The rate of growth of stem or root tips can also be measured by the 

 very simple method of marking the organ with short horizontal lines spaced 

 equidistantly. The marks are generally made with India ink applied with a 

 fine brush. The rate of elongation is determined by observing the position 

 of the marks after the lapse of a definite period of time. A similar method 

 can be used for measuring the rate of increase in the area of a leaf by ruling 

 a cross-sectional pattern of lines on the leaf. Such methods are of value in 

 indicating in what part of the organ enlargement is occurring most rapidly. 



A number of different types of instruments have been devised for the 

 automatic measurement of the rate of height or diameter growth in plants. 



Accumulation of Foods. — Although the simple carbohydrates synthesized 

 in photosynthesis may undergo many transformations the sum total of the food 

 available to a green plant can never exceed the amount produced in photosyn- 

 thesis. A large proportion of the photosynthate is normally consumed in the 

 processes of assimilation and respiration. Any surplus which remains accumu- 

 lates in one or more tissues or organs of the plant. Accumulation of foods, how- 

 ever, does not occur continuously. For considerable periods in the life cycle of 

 most species not only is no accumulation of food occurring, but a more or 

 less rapid consumption of food reserves is in progress. In woody plants dur- 

 ing the dormant season slow utilization of food in the processes of respiration 

 and assimilation continues. When growth is resumed in the meristematic 

 tissues of such species in the spring, there is always a considerable drain on 

 the supply of foods stored in the plants since much of this growth is accom- 

 plished before the photosynthetic rate is rapid enough to compensate for the 

 necessarily speedy utilization of food which occurs. Similarly the sprouting 

 of bulbs, corms, tubers, rhizomes, etc. always occurs at the expense of the 

 stored foods in such organs. The same is true of seeds when germination 

 occurs. Much of the food which accumulates in plants during periods when 

 photosynthesis exceeds the food-consuming processes is utilized by the plant 

 sooner or later in its life history. 



The organs in which most accumulation or "storage" of food occurs are 

 different in different species. In annuals food storage occurs predominantly 

 in the seeds. Foods also accumulate in the seeds of most biennial and perennial 



