GROWTH 217 



whole tissues will be irregular, causing the curvature of 

 whole organs. But similar unequal growth may also be 

 manifested in different parts of one and the same cell, 

 according to the distribution of the protoplasm and 

 the nucleus, as we have already seen. It has, in fact, 

 been noticed that whenever single cells curve, accumula- 

 tions of protoplasm become noticeable on the concave 

 side. This side is probably more thickened, and offers 

 greater resistance to the osmotic pressure of the sap, 

 and gets less distended. We may add that it is not the 

 thickness of the cell-walls alone which figures in the 

 phenomena of growth : the chemical and physical pro- 

 perties of the cell-wall may vary, and make it more or 

 less resistant and elastic. It has been proved that a 

 certain ferment exists which softens the cellulose of the 

 cell-wall. The local appearance of this ferment can 

 influence the direction of the growth of the cell and its 

 external outlines. Perhaps these facts will furnish us in 

 their turn with a clue to the explanation of another of 

 Darwin's observations which we mentioned before 

 his observation concerning decapitated roots. Very 

 possibly during the cell's earliest period, when it is 

 consequently lying close to the root-tip, there is an 

 irregular distribution of the protoplasm and the nucleus, 

 etc., which may result in a subsequent irregular growth 

 and a curvature of the whole organ. 



Only now can we fully estimate the endless variety of 

 the effects of external conditions upon the growth of 

 organs, tissues, cells, or even parts of cells, and the com- 

 plicated combinations all these phenomena may present 

 when taken together ; and only now can we realise what 

 a mistake is made when, instead of trying first of all to 

 find out these possible explanations, people make up 

 their minds that in the phenomena of growth are to be 

 seen the results of some psychical, almost conscious, 

 activity of the plant. 



