GRO WTH. 1 83 



the uncut stem, while the outer ones were slightly stretched. 

 The strains thus set up are spoken of as longitudinal tissue 

 tensions. Similar tensions due to unequal transverse growth 

 may be shown to exist. If a thin transverse slice from the 

 fleshy leaf-stalk of the rhubarb be divided into equal parts by 

 a longitudinal cut it will be found in a few moments that the 

 halves can no longer be made to touch throughout the line of 

 the cut, because it has become convex. Both the longitudi- 

 nal and transverse tensions may be exaggerated if the parts 

 be placed for a few moments in water. 



260. Conditions of growth. — That plants may grow cer- 

 tain conditions are prerequisite. (1) There must be an ade- 

 quate supply of constructive materials. These may be derived 

 either from food recently manufactured or from that stored 

 in reservoirs, or, in the case of the colorless plants, from that 

 absorbed from without. (2) There must be a supply of oxy- 

 gen for respiration. This is needed, as previously explained, 

 to set free the energy necessary for growth. (3) There must 

 be a supply of water adequate to maintain a minimum turgor 

 of the cells, without which growth cannot take place. (4) 

 A suitable temperature is required. The range of temperature 

 within which growth may take place is extensive, and varies 

 with the individual plant. In general, the upper limit may 

 be stated as about 40 C, and the lower, about o° C. The 

 minimum of plants of tropical regions is approximately io° C. , 

 while the maximum for plants of the arctic or alpine regions 

 is much below 40 C. Between the maximum and minimum 

 temperatures there is an optimum temperature for each plant, 

 at which growth takes place most rapidly. For most plants 

 the optimum lies between 25 b and 32 C. 



261. External conditions exercise a very important in- 

 fluence upon the rate or character of growth by reason of the 

 irritability of the protoplasm. (See further ^[ 418.) Many 

 of these conditions act upon members of the plant so as either 



