570 GROWTH, ASSIMILATION, AND ACCUMULATION 



species. During the process of germination the embryo uses food that was 

 made by the preceding sporophyte generation. 



Most of the accumulation of food in typical biennials such as beet, carrot, 

 parsnip, turnip, etc. occurs in fleshy roots or root-like structures. This ac- 

 cumulation of food by biennial species occurs mostly during their first season's 

 growth. During the second season most of the accumulated food is utilized 

 in the production of flowers, fruits, and seeds so by the end of their life cycle 

 the vegetative parts of such plants contain relatively little food. 



In perennial species considerable storage of food often occurs in seeds 

 and fruits but the principal organs of food accumulation in many species 

 which live for a number of years are the stems and roots. In woody species 

 the pith, cortex, vascular rays, and wood parenchyma are the stem and root 

 tissues in which most of the accumulation of surplus foods occurs. Modified 

 stems such as rhizomes (iris, many ferns, Solomon's seal, etc.) tubers (potato, 

 Jerusalem artichoke, etc.) corms (crocus, gladiolus, jack-in-the-pulpit, etc.), 

 and bulbs (onion, tulip, hyacinth, etc.) are almost invariably regions of food 

 storage in species which possess such organs. 



The great bulk of all foods which accumulate in plants can be classified 

 into the familiar categories of carbohydrates, fats and proteins. 



The principal storage carbohydrates are starch, sucrose, "hemicelluloses" 

 and inulin (Chap. XXII). Accumulation of oils (fats) in abundance occurs 

 most commonly in seeds, although such compounds are stored in at least small 

 quantities in the cells of many tissues. Proteins, like fats, accumulate prin- 

 cipally in seeds. 



The cells of "storage" tissues are not merely passive reservoirs in which 

 excess foods pile up. Foods move into the cells in which they accumulate 

 only in the soluble form, yet with few exceptions, sucrose being the only im- 

 portant one, the foods amassed in storage cells are converted into an insoluble 

 form. Upon translocation into storage cells, glucose is converted into starch, 

 amino acids into proteins, fructose into inulin, fatty acids and glycerol into 

 fats, etc. All of these chemical transformations are condensation reactions 

 which are catalyzed by enzymes occurring in the living cells in which the 

 foods accumulate. 



Conversely, stored foods cannot be utilized by any part of a plant until 

 they have first been digested into soluble forms as a result of enzymatic 

 activity. Until such a transformation has occurred they cannot be translocated 

 out of the cells in which they are situated into the cells in which they are 

 utilized in assimilation or respiration. 



