74 CARNEGIE INSTITUTION OF WASHINGTON. 



under more equable conditions than that to which the plants in the open 

 were subjected. 



The course of desiccation under the conditions named has already 

 been described in several papers. Certain features in the variations 

 in weight of the plants under observation, however, remained without 

 adequate explanation. Among these is to be included the fact that 

 the rate of water-loss decreases more rapidly than the ratio of succu- 

 lence, or proportion of water present to the area of the transpiring 

 surfaces. 



Among the survivors of the original lot of plants taken for the test 

 was one large Echinocadus which had been taken from the soil in 

 November 1908 and kept in a shaded room for more than 6 years. 



The following conclusions as to the water relations of such large 

 succulents are established by the examination of this material: 



Echinocadus in diffuse light may lose as much as one two-thousandth 

 part of its weight in one day, immediately following the excision of its 

 root-system. The same plant 6 years later, under equivalent condi- 

 tions, except that its weight had been reduced nearly a third, lost no 

 more than 1 part in 17,000 of its weight in one day. 



An Echinocadus weighing 38 kg., of which 90 to 95 per cent may be 

 estimated as water, lost 3.5 kg., or one- tenth of its total water, in the 

 first year of isolation in diffuse light. In the sixth year the loss was 

 one-twentieth of the water-supply at the beginning of that year. 



Echinocacti in the open lost 38 to 45 per cent of their original weight 

 during the period from June to November inclusive. Individuals 

 in the diffuse light of the experimental rooms lost 7 or 8 per cent in the 

 same period. 



Echinocadus is capable of growth in the apical region, in plants in 

 which water-loss and disintegration of the carbohydrates (including 

 hydrolysis of the cortical walls) has reached an advanced stage. 



The rate of loss in weight of an Echinocadus, largely due to evapora- 

 tion, is not correlated with the degree of succulence (proportion of 

 amount of water present to superficial area of body) or with the density 

 of the sap, but is to be attributed to morphological causes. 



The difference in behavior of Echinocadus and Opuntia in desic- 

 cation and starvation is correlated with definite physical features. 

 Echinocadus has a globoid stem consisting largely of thin-walled cells, 

 in which the accumulated food-material is in the form of soluble carbo- 

 hydrates. Solid material and accessory colloids are noticeably lacking. 

 The flattened joint of Opuniia is composed of a network of fibro- 

 vascular tissue. The fundamental tissue is rich in slime or mucilage, 

 and somewhat higher in total hj^drolyzable carbohydrates than is the 

 fibro-vascular tissue. The loss of water from the large, globose stems of 

 Echinocadus is much more affected by illumination than in the flat- 

 tened stems of Opuntia. The course of respiration in the thin stems of 

 of Opuntia is such that acids formed during the process are present 



