720 ECOLOGY 



corms and tubers is that their food supply enables them to renew 

 activity speedily at the inception of a favorable period, a matter of 

 much significance in climates like that of Italy, where short favorable 

 periods are intercalated between long periods of inactivity. 



The role of accumulated foods. The plant that accumulates the 

 food may itself utilize it; for example, during the vegetative season the 

 trunks of deciduous trees gradually accumulate quantities of starch 

 and other foods, which are utilized in the development of shoots the 

 following spring; or the accumulated food may be utilized by vege- 

 tative offshoots, as in plants that give rise to bulbs and tubers. Again, 

 the food accumulated in seeds may be used by the progeny of the ac- 

 cumulating plant. Finally, accumulated food may be usec' by a 

 foreign organism or even remain unused. Non-utilization is more fre- 

 quent than usually has been supposed. Most tubers and bul'>s and 

 some seeds contain more food than is used under ordinary conditions 

 by the germinating plant, though it is probable that an excess beyond 

 the amount necessary to support the plant until it reaches the light is 

 beneficial in the way of giving it a " running start." 



In the potato tuber there is a large excess of food beyond the amount utilzed in 



germination (fig. 1037). In various orchids 

 (as in Neottid) the amount of accumulated 

 food which is utilized is small compared with 

 that which is left; after a time such food de- 

 cays and contributes to the humus. The large 

 quantities of food accumulated in galls are value- 

 less to the accumulating plants. Latex contains 

 starch and other foods that never are utilized, so 

 far as is known. During the summer and autumn 

 there is a well-marked migration of food from 

 FIG. 1037 -A tuber of the the leaves to the trunk in deciduous trees; how- 

 potato (Solanum tuoerosum). ' .. . . , . 



which has germinated in a dark, CVer > cons ' derable fo d rema ' n * ' n the leaves at 

 moist cellar; note the wrinkling of leaf fa " and hence never is ^'Hzed. 

 the tuber, evidencing the with- The detailed consideration of the structure 



drawal therefrom of food and and arrangement of plant foods is deferred to 

 water by the developing shoots. the chapter dealing with seeds (p. 911). 



Latex. The structural features of latex elements. A few plant fami- 

 lies are characterized by the presence of milky juice, or latex. Geneti- 

 cally, latex elements are of three sorts, the simplest being the latex sac, 

 where the milky juice is contained in uninucleate cells, usually arranged 

 in longitudinal rows, as in Sanguinaria, and also in the Convolvulaceae 



