916 ECOLOGY 



in the neighborhood of 25 C. In cold weather, sugar is not readily transformed 

 into starch; indeed, the reverse process often takes place (as in the sweetening of 

 potatoes). An important variable in starch production is the supply of available 

 sugar; if the sugar concentration is high, starch forms more rapidly and at lower 

 temperatures than usual, even at o C. Indirectly, light favors starch formation in 

 that it induces a considerable production of sugar, from which starch can be made, 

 but the latter can form in the darkness as well as in the light. The influence of 

 external factors upon the accumulation of fats, proteins, and " reserve cellulose " 

 is not known. For a consideration of food accumulation in tubers and galls, see 

 pp. 719, 782. The role of food in seeds will be considered in connection with 

 germination (p. 934). 



The structure of the food-containing cells. Practically all cells in which food 

 accumulates, whether in the endosperm, perisperm, or cotyledons (and also in galls 

 and tubers), are parenchymatic and also are thin-walled except in those cases where 

 the food accumulates in the walls rather than in the lumina as in " reserve cellu- 

 lose "). There are protoplasmic connections between adjoining endosperm cells, 

 and where the walls are thick, as in " reserve cellulose," the canals containing the 

 connecting protoplasmic threads are quite conspicuous (fig. 1208). 



Variations in seeds and fruits in relation to external factors. The 



fusion of gametes in relation to fruit development. Were the phe- 

 nomenon not so universal, it would seem amazing that large fruits are 

 able to develop as a result of so slight an external stimulus as that in- 

 troduced by a male gamete upon fusion with an egg. In general those 

 pistils in which this fusion takes place develop into fruits, while other 

 pistils show no such changes, soon dropping off, as do the stamens. 

 In the simpler cases fruit development involves only the enlargement 

 or elongation of the ovary, but in other cases various organs may be in- 

 volved, for example, the calyx and the receptacle (as in the apple). 

 Sometimes the stimulation appears greater in the case of xenogamy than 

 of autogamy; for example, in Cheiranthus the fruits are twice as large, 

 and the seeds are heavier and more numerous on the cross-pollinated 

 individuals. 



There is much in common in the formation of fruits and galls, and in each case 

 it has been held by some investigators that the growth arises solely through the 

 influence of a momentary stimulus at the inception of the process, and by others 

 that the activities within the growing structures afford constant stimuli for further 

 development. Of interest in this connection is the fact that staminate flowers 

 may be transformed into galls if stimulated by the proper insects (as in the ash). 

 In this event, instead of dropping off, they enlarge and remain for a year or more. 

 Here a foreign stimulus given by the gall insect causes the retention and the further 

 development of the staminate flowers much as another foreign stimulus given by 

 the male gamete more commonly causes the retention and the further development 

 of the pistillate flowers. The precise nature of the fruit-forming stimulus varies 



