BOOKS AND CURRENT LITERATURE 117 



Guayule with its deep root system would not find soil moisture conditions 

 favorable in a heavy soil, when the amount of rainfall is only 7 to 10 

 inches, but would thrive on the gravel. Moreover it is doubtful if lack 

 of aeration is a factor in arid soils, and until the soil moisture conditions 

 are known, especially for the deeper layers of the soil, it seems unsafe to 

 ascribe soil preference to lack of aeration. By the application of the 

 quadrat method first described by Clements (Clements' name' is, how- 

 ever, not found in the bibliography), Lloyd has given us two typical quad- 

 rats which should be of great value as permanent records of the vegeta- 

 tion of this great area. In this section of the work there are also given 

 some interesting conclusions bearing on primary succession. The con- 

 clusion is reached that Parthenium cannot compete successfully with 

 Agave lecheguilla, which therefore becomes the most objectionable of the 

 native plants in so far as the natural crop of Parthenium is concerned. 

 The principal enemies of the Guayule are rust, dodder and grazing 

 animals. A bark beetle seriously damages the crop after harvest. The 

 Guayule plant varies greatly in size. Usually a plant weighs from 500 

 to 600 grams and is from 40 to 50 cm. high. The greater number of the 

 leaves which are developed during the period of sufficient moisture 

 supply fall off during the dry period, only the clustered leaves at the end 

 of the branches remaining until the following year. Leaf fall is there- 

 fore the direct effect of drought. 



The greater part of the monograph is given to the histology of the 

 plant. This is treated in considerable detail. The effect of irrigation on 

 the anatomy of the plant is discussed in many places throughout the 

 publication. It is found that starch remains in the endodermis much 

 longer in irrigated than in unirrigated plants, also that more wood is 

 developed in irrigated than in unirrigated plants. The medullary rays 

 are narrower and there is less cortex. In other words the elements hav- 

 ing to do with the conduction of water are much better developed in 

 irrigated plants which supposedly have the greater amount of transpira- 

 tion. The annular structure so distinctly seen by the eye, is not a true 

 annular structure, nor is the true annular structure to be correlated with 

 years of growth, since it represents only periods of growth which may or 

 may not occur each year. On this account it is extremely difficult to 

 estimate the age of plants and this difficulty is even greater with irri- 

 gated than with unirrigated plants. The resin canals are grouped in 

 six primary and one secondary system and are of especial interest in con- 

 nection with the formation of rubber in this plant. Rubber is not formed 

 as a latex as in the majority of rubber plants, but is laid down in the cells 



