48 



THE CARBOHYDRATE ECONOMY OF CACTI. 



V. SEASONAL VARIATIONS IN THE CARBO- 

 HYDRATE-CONTENT. 



The vegetation of the desert is exposed to extreme climatic variations. 

 It is not any single factor that determines the features commonly ascribed 

 to the desert, but rather a great complex of conditions. Thus, while scanty 

 water-supply may be of greatest importance, it is not only the low average 

 rainfall which makes for aridity, but the distribution of this precipitation 

 throughout the year, as well as the nature of the soil and its drainage. To 

 these must be added other conditions, such as the low relative humidity of 

 the air and the high winds, all favorable conditions for water-loss, great 

 diurnal variation in temperature, and intense sunlight. 



In table 11 is given the 

 monthly precipitation during 

 1916 and 1917, the years in 

 which these investigations were 

 carried out. 



In this consideration rainfall 

 is, of course, of importance only 

 as it replenishes the moisture of 



TABLE 11. 



the soil as a supply for the plant. 

 Cannon 1 has shown that the absorbing roots of these cacti lie at a depth of 

 10 to 20 cm. It is, therefore, rather a consideration of soil moisture at 

 this depth which is of immediate importance. Shreve 2 has analyzed the 

 relation between rainfall and soil moisture in the region from which the 

 plants used in the present investigation were taken. There are two well- 

 defined periods of rainfall during the year the winter rains, December to 

 March, and the summer rains, July to September. For the 34 years' rain- 

 fall record of Tucson, Shreve calculated the distribution of rainfall during 

 the two rainy seasons and the intervening time. The percentages of the 

 annual total are: winter 31.7 per cent; arid fore-summer 5.9 per cent; 

 humid mid-summer 50.7 per cent; arid autumn 11.7 per cent. The sea- 

 sonal variation of soil moisture during the year runs closely parallel to the 

 significant periods of precipitation. The same conclusion may be arrived 

 at from a study of the graphs prepared by Livingston 8 of the relation of soil 

 moisture to desert vegetation. This well-defined seasonal distribution of 

 precipitation gives to the vegetation a very marked periodicity of activity.' 1 

 Many of the reactions which have been observed as characteristic of these 

 seasons have their origin in the more deep-seated metabolic activities in- 



1 CANNON, W. A. The root habits of desert plants. Carnegie Inst. Wash. Pub. No. 



131, 43-52, 1911. 

 3 SHREVE, F. Rainfall as a determinant of soil moisture. The Plant World, 17, 9-26, 



1914. 

 3 LIVINGSTON, B. E. Relation of soil moisture to desert vegetation. Bot. Gaz., 50, 



241-256, 1910. 

 * MAcDoiiGAL, D. T. The course of the vegetative seasons in southern Arizona. The 



Plant World, 11, 189-201, 217-231, 237-249, 261-270, 1908. 



