144 Hydration and Growth. 



cient to have a distinct effect on the hydration capacity of the cell- 

 colloids. Late in the daylight period the acid may be reduced to a 

 point below the hundredth normal which has been used so exten- 

 sively in this work as a standard solution. The effects of acidosis 

 would, of course, vary as the composition of the pentosan-protein-salt 

 colloid of the joint passed from its embryonic aspect to that of the 

 mature member. The actual amount of water lost by the greater 

 number of plants, especially those with thin stems and broad leaves, 

 is greater for the daylight period than for the night. Opuntia is a 

 notable exception to this generalization, and its rate of transpiration 

 is greatest during the night, usually between midnight and morning. 

 All of these agencies affecting imbibition likewise have a determining 

 influence on growth, and the resultants in Opuntia and presumably in 

 other massive succulents are such as to constitute a characteristic 

 type of growth. 



Another feature about which but little has been said is that of the 

 morphology of the compound members of the stems of Opuntia. At 

 first, when a young joint is but 2 or 3 cm. in length, it is wholly in an 

 embryonic condition and its colloids show the reactions of such mix- 

 tures. As development progresses much permanent tissue is formed, 

 in which finally the embryonic tracts lie as a network or reticulum. 

 Any given section of a growing joint contains growing and mature 

 tissue after a certain stage is reached, but when the mature tissue 

 reaches a proportion something greater than that of the growing 

 masses its characteristic variations in hydration overshadow those of 

 the growing cells and give rise to the retardations and shrinkages 

 which are so marked a feature of these plants. It is probable that 

 many features of the rate and course of growth may be ascribed to 

 anatomical relations, of which the above is an illustration. 



