112 STRUCTURE AND FUNCTIONS OF LEAVES 



134. Resting condition and diminished respiration. The 



whole plant body or parts of it may pass into a resting condi- 

 tion, in which growth is suspended and few manifestations of life 

 are discernible. Familiar examples of this inactive condition are 

 leafless trees in winter, and rootstocks, tubers, and bulbs during 

 the winter of ordinary temperate climates or the rainless sum- 

 mer of southern California and the Mediterranean coast region. 

 Seeds and many kinds of resting spores afford extreme instances 

 of the possibility of a suspension of activity for years, followed 

 by prompt growth when suitable conditions are supplied. In 

 general, a moderately low temperature and dryness favor the 

 resting state. During the resting period respiration is greatly 

 diminished, so much so in the case of thoroughly dry seeds as 

 to be almost or quite imperceptible. 



When resting protoplasm is placed in circumstances which 

 enable it to begin active respiration, growth and development 

 soon appear. Thus twigs of lilac or other shrubs will flower 

 after a time, when placed in water and brought into a warm 

 room in winter. 



In many cases, as with most seeds, the period of repose is 

 essential for growth. Potato tubers will not sprout as soon as 

 they are mature: some varieties need only two months and 

 others four or five months of rest. 



135. Assimilation. By most American plant physiologists l 

 the word assimilation is used as a name for the series of changes 

 by which the plant transforms absorbed or manufactured food 

 into the materials of its own tissues. 



The transformation of starch or sugar into substances, like 

 cellulose, which consist of the same elements (carbon, hydrogen, 

 and oxygen) differently combined, is a relatively simple matter ; 

 but the manufacture from carbohydrates of such very complex 

 nitrogenous substances as the proteids and living protoplasm 

 is a most complicated process, and imperfectly understood. 



1 European botanists often include in the term assimilation both photo- 

 synthesis and the processes discussed in this section. 



