36 



THE WATER-BALANCE OF SUCCULENT PLANTS. 



October 20 it had gained 10.685 kg", in weight and 29.5 cm. in circum- 

 ference. The proportion of loss in weight and circumference to the gain 

 in the same do not agree very well, the loss in weight being 403 grams 

 per centimeter of circumference and the gain 368 grams per centimeter. 

 It must be remembered, however, that the loss was not exactly known, but 

 was estimated for 29 days since the last weighing (p. 61). 



The measurements of Bchinocadus , so far as they have been carried, 

 show: (l) the same principles in general govern its expansion and con- 

 traction as in the case of the sahuaro; (2) measurements of a part of the 

 circumference are not necessarily proportional to the whole; (3) measure- 

 ments of the entire circumference may agree almost exactly with changes 

 in weight, but the changes in any one dimension, in a plant where the 

 changes of form are more or less local, can not be relied upon to bear 

 exact proportion to changes in weight. The only safe method of drawing 

 conclusions from such data is in the comparison of a large number of meas- 

 urements carried through a cons'iderable period of time. 



GROWTH. 



From the great difference in the morphology of Carnegiea and Echino- 

 cadus it is plain that their growth must be very unlike. Both start in much 

 the same form, but one may become a branched column 15 meters in height, 

 while the other never branches and rarely attains the height of over a 

 meter. 



Table 9. — Rate of vertical growth of Echinocactus wislizeni. 



*Tliis was only approximate, as the original mark near the base could not be found. It is 

 probably within 2 cm. of the correct measurement. 



The younger plants seem to grow almost equally in all directions, as they 

 assume an almost globular form, which continues until they are about 30 cm. 

 high. Occasionally a plant is found which seems to have grown faster 

 laterally than vertically, since its diameter exceeds its height; but plants 

 over about 30 cm. high begin to assume a cylindrical form with the apex 

 facing the south. The growth in length is apical, and the increase in cir- 

 cumference is provided for by the insertion of new ribs from time to time. 



