GRO WTH. 1 59 



wide, slitting it on one side only, taking care not to stretch it. Keep 

 it in a moist atmosphere for a few minutes, and then replace it. Does 

 it meet about the wood ? 



B. Cut a slice about 2 mm. thick from the end of a stalk of rhubarb. 

 Bisect this and keep the halves for a few minutes in a moist atmosphere, 

 then place severed edges together. Do they touch throughout ? 



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

 tain conditions are prerequisite, (i) There must be an 

 adequate supply of constructive materials. These may be de- 

 rived 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 oxygen 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 supply the mate- 

 rial for filling the cells during the phase of enlargement. 

 (4) A suitable temperature is required. The range of tem- 

 perature 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 0° C. 

 The minimum of plants of tropical regions is approximately 

 10° 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° and 32° C. 



220. External conditions exercise a very important influ- 

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

 irritability of the protoplasm. (See further "[y 317.) Many 

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

 to bring about permanently unequal growth in a certain part, 

 or to cause one part to grow more or less rapidly for a time 

 than another. Such variations in growth produce curvatures 

 in the parts concerned and move members connected with 

 them. They are discussed in the chapter on Movements. 



