Periodical Literature 749 



growth interrupted by periods of rest, by varying the supply of 

 mineral salts and light intensity at the disposal of the plant. The 

 functions of various mineral salts as determined by Hansteen and 

 others are explained, from which it appears that calcium, to which 

 certain amounts of potash and magnesium salts are added, is the 

 most important factor in forming cell walls and also in enabling 

 roots to take up minerals by chemical reactions (not simply by 

 osmosis). Hence the importance of Ume in soils. The necessity 

 of an active water circulation to carry away chemical products 

 so as to permit further chemical production is accentuated. 



Water circulation is reduced not only by dryness of soil, but by 

 decrease in temperature and reduction of light intensity and light 

 duration. The fall of leaves and final rest of vegetative functions 

 is simply explained by the change in exterior conditions. 



The species with leathery, thick-skinned leaves (conifers) can 

 inhibit their functions or resiune them at any time without change 

 in form, because a small water supply suffices for them. The 

 water-needing species, on the other hand, respond to a longer 

 withdrawal of water by loss of the assimilating organs and only a 

 decided change of exterior conditions, especially a lengthening of 

 light influence, causes new formation of foliage. By influencing 

 these exterior conditions within narrow limits at any season without 

 any period of rest a continuation of growth can be secured. There 

 is no "need of rest" where assimilating organs are renewed, and 

 not as in the animal used up. Plants die from hunger and thirst, 

 animals from age, being used up. 



Under the caption, "The specific form of the tree as product of 

 diameter growth," the author combats Metzger's theory of the 

 wind as determining factor of the form and structure, namely 

 such as offers the greatest resistance with the smallest amoimt of 

 material. Elsewhere, the author believes to have proved that 

 "the stem of an old spruce in timber forest, in which the dying of 

 the crown basis takes place as the tip grows, corresponds to a shaft 

 of equal conductivity for water, and not to one of equal resistance 

 (to bending) for its whole length." Every stem shows varying 

 resistance to bending at different heights, as shown by calculations. 

 The big trees, if soimd, show in proportion to their crown develop- 

 ment a much greater resistance than younger, thinner stems. 

 Trees work with surpluses and use much more material for the 

 building up of their stems than resistance to the ordinary wind 



