MacDOUGAL— HYDRATION AND GROWTH. 371 



It would be unwise to assume, however, that the general pro- 

 cedure followed by the walnut, tomato and by some green stems, is 

 universal. 



The growth of the flat joints of Opitntia which were described 

 before this Society in April, 1917, presents many features different 

 to the above, and some tracings of the growth record of Opuntia 

 discata for three weeks are shown in figure 3. 



Enlargement (denoted by a downward course of the recording 

 pen) begins in the morning and continues with the rising tempera- 

 ture until mid-afternoon, then slows down and shrinkage sets in 

 which continues through the night. Such shrinkage must inevitably 

 accompany and result from excessive water-loss, and in confirma- 

 tion it is found that the cacti show the greatest transpiration at 

 night, at which time the acidity rises until it is ten times as great as 

 in the daytime. The disintegration of this by light and higher tem- 

 peratures increases the imbibition capacity of the cells known to be 

 high in pentosans and a swelling or growth in the daytime results, 

 producing a growth record almost exactly in reverse of that of the 

 walnut and tomato. 



The rate, course and amount of growth are at all times a re- 

 sultant of agencies which affect water-loss, hydration capacity, 

 respiration and its residues and other features of metabolism. 



The procedure in transpiration from the surfaces of a growing 

 organ may be such that the maximum loss partly masking growth, 

 may for example occur in the cacti at night, while it comes in the 

 midday period in the commoner types. The swelling or hydration 

 capacity of any plasmatic mass which determines its capacity for 

 growth or enlargement depends in the main on the mucilages or 

 pentosans present and their amount relative to the proteins, the 

 character of which also affects growth. 



The biocolloids of the plant show a degree of swelling in water 

 greater than that in solutions containing free hydrogen ions, so that 

 growth generally is most rapid in cell solutions near the neutral 

 point. In modification of this last statement, it is to be pointed out 

 that maximum swelling effects may result from the action of some 

 of the amino-acids. 



Rises in temperature within the range ordinarily associated with 



