350 PHYSIOLOGY OF NUTRITION. 



necessary to clear the sections ; they may, however, be cleared 

 if requisite by glycerine. 



If we examine in this way branches several years old of Alnus, 

 Betula, Acer, Syringa, etc., in summer, we find large quantities of 

 sugar in the vessels. The wood fibres and the living wood 

 elements (the parenchymatous cells of the wood and medullary 

 rays) are for the most part free from glucose. In the vessels of 

 the thicker leaf nerves likewise no glucose is present, but it is 

 present in the parenchyma surrounding the nerves, and acting as 

 conducting sheath. At the time of the autumnal starch maxi- 

 mum the distribution of the glucose is nearly the same as in the 

 summer. 



When the fall of the leaves has set in and the autumnal starch 

 maximum has come about, solution of the starch in the steins and 

 branches of our trees and shrubs at once begins. This gives rise 

 to the winter starch minimum, which may be complete, say, in 

 December, and lasts till about the beginning of March. In many 

 cases (Salix, Quercus, Corylus, Syringa) the wood, at the time of 

 the winter starch minimum, still contains much starch, while the 

 starch has disappeared from the cortical tissue, and probably has 

 migrated into 'the deeply lying parts mainly in the form of 

 glucose, which moreover in winter, as in summer, is present in 

 the cortex (starchy trees). If, on the other hand, we examine 

 branches of Tilia or Betula in January, we find starch neither in 

 the wood nor in the cortex (fatty trees). It has been transformed 

 into fat, which can easily be detected in the tissues on treating the 

 sections with alkanet tincture (see 121). 



In Tilia, e.g., the medullary rays and inner cortical tissues 

 in particular are fatty in winter, while in autumn they contain 

 much starch. In the pith of Tilia glucose is abundantly present 

 in the winter, besides fat, while particularly the peripheral part 

 of the pith contains in the autumn much starch. 



We now continue our experiments by testing branches of 

 Corylus and Tilia, in April, for starch. We again find starch 

 both in the wood and in the cortex. This has clearly been re- 

 generated from fat and glucose. The process of reconversion 

 commences at the beginning of March (in fatty trees at first in 

 the cortex) ; it continues till about the "end of April. At this time 

 the spring starch maximum is attained. 



When the leaves begin to unfold, the starch again passes into 

 solution, ind in this way is brought about the starch minimum of 



