MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 325 



greater number of instances particularly in Triticum, no such ex- 

 planation could be deemed adequate, and the matter is referred to 

 varying imbibition capacity coincident with alternations of acidity, 

 alkalinity and neutralization (see page 309). 



The highest rate that was maintained for some time by Zea was 

 found to lie between 2"/° C. and 30° C. The elongation of the leaf 



9 10 II MT I 2 3 



mm 



1 1 1 TtH 



Fig. 6. Auxographic record of growth of leaf of wheat {Triticum) for 

 six hours showing sudden alterations in rate of elongation. The pen moves 

 downward with elongation. Actual variation in length. X I5- 



of Triticum was erratic and retardations were numerous and occur- 

 ring at all temperatures between 15° C. and 30° C. It is not pos- 

 sible to fix upon any limits of temperature within which growth 

 might be continuous in this plant. It is obvious that " secondary " 

 maxima might readily be derived from data of this character. 



No retardations occurred except after 11 A.M. in either Zea or 

 Triticum and while Zea showed an acceleration late in the day after 

 retardation at high temperatures, Triticum did not. The tonic range 

 of the two plants is of course not identical. Wheat grows at a 

 lower range than corn and probably reaches its upper limit near 

 the figures given. 



There are but three allowable causes in the present state of our 

 knowledge, to which might be attributed the slackening or inhibition 

 of growth or actual shrinkage of growing joints after midday and 

 continuing until the following morning. The retardations in ques- 

 tion are relatively least in the earlier stages of development when 

 the joints are not more than one fourth or one fifth adult size and 



