AN ARTIFICIAL OSMOTIC CELL 47 



ten hours. At 42°C. they grow at the rate of 2.5 cm. per hour 

 during the first twenty-four hours and at the rate of 30 cm. per 

 day during succeeding days. 



Growth is more rapid when the stem clings to the side of the 

 vessel than when it is surrounded on all sides by the solution. 

 The vessel wall (or glass plate, etc.) supports the inner layer of 

 the stem as it is formed. The outer and middle layers are not 

 then formed, as has been remarked; that is, the side of the stem 

 in apposition to the solid surface is imperfect. Thus not so 

 much KMn04 is used in forming the wall, and a more rapid rate 

 of growth is the result. When stems are crowded in a limited 

 space to the extent of interfering with each other's growth, this 

 factor — the economy of substance — makes for the persistence 

 of the stems situated in apposition to a foreign body, at the 

 expense of others, which cease growing. This question will be 

 further discussed under a separate heading. 



Rapidity of growth being in direct proportion to the degree 

 of osmotic pressure within the sac, it is necessarily less rapid 

 when a number of stems arise from the same sac than when only 

 a single stem is formed. 



DIRECTION OF GROWTH 



When the conditions of the environment are the same on all 

 sides of the stem, its direction is vertically upward, this being 

 due to the fact that the solution within is less dense than the 

 surrounding medium. If, in the course of growth, the vessel 

 be tilted, turned on the side, etc., the stem bends and still pro- 

 ceeds to elongate upwards. In this manner a stem may be 

 made to assume almost any linear figure (see fig. 2, D). If 

 a horizontal obstacle be placed above the vertically growing 

 stem, the latter will branch out laterally along the under sur- 

 face of the obstacle, until one of the branches will have reached 

 its edge, when it will continue vertically upwards again. 



When the stem is surrounded on all sides by the solution, 

 it is rarely perfectly vertical. This is due to the extreme sensi- 

 tiveness of the stem to differences in temperature on opposite 

 sides. After the stem has been formed, it does not respond to 



