196 Reserve Material of Plants. 



of June, or excessive accumulation of starch in the bark during 

 active growth, they at once become certain and most important 

 indications of disease. 



As in the early period of vegetation, the nutritive and assimi- 

 lative functions are nearly balanced, there is no surplus material 

 to be deposited. As the season advances, growth diminishes and 

 the products of assimilation then become in excess, in which case 

 they are stored up — most generally in tissues which ha^e long 

 since lost their power of growth — and are therefore designated as 

 permanent, though sometimes in tissues still active, but 

 specially modified as reservoirs of reserve material. These 

 reservoirs represent different tissues and organs in differ- 

 ent plants. In the potato, it is the tuber itself; in the lily, it is 

 the modified leaves forming the scale of the bulb ; in the carrot, it 

 is the root ; and in the century plant, it is each leaf, which becomes 

 specially modified for that purpose. Our particular purpose, how- 

 ever, will be best illustrated by confining our attention to trees 

 and other plants in which the woody structure is largely in excess. 

 Here the distribution of the starch for storage is determined 

 first of all to the pith, next to the medullary rays and woody 

 cells, and last of all to the bark, thus being correlated to the ac- 

 tivity of the tissues themselves. In trees, the deposition of starch 

 may be regarded as commencing somewhat late in the season, and 

 increasing, as growth diminishes, to the time when the function of 

 the leaves ceases. 



No law can as yet be stated concerning the amount of starch 

 which should normally be deposited in the various tissues, but as 

 the result of examinations into the histological condition of 

 several thousand specimens taken from a great variety of trees 

 and shrubs, examined at all seasons of the year, the following 

 conclusions appear to be justified : — 



1. While, in general, plants store reserve material at the close 

 of the growing season, this law cannot find specific application 

 in all cases and for all tissues. 



2. Woody plants generally contain reserve material in their 

 permanent structure during the period of active growth, but the 

 presence of starch in the cortical tissues during this time is vari- 

 able in different species, and depends upon the special physiologi- 

 cal functions of the subject. 



