118 BOOKS AND CURRENT LITERATURE 



themselves. It occurs in the parenchyma cells of the stems and roots 

 and is secreted most rapidly in the cells of the resin canals, first in the 

 roots and later in the stems. It usually appears in the tissues sometime 

 after the cessation of active growth. The amount of rubber produced 

 varies from 3.3 per cent in irrigated to 9.7 per cent in unirrigated plants. 

 While the cortex ©f irrigated plants contains 4 per cent, that of unirri- 

 gated plants contains 15 or 20 per cent. Although the percentage of 

 rubber produced is less in irrigated plants it is estimated that when one 

 considers the increased growth, as much rubber if not more is produced 

 than under natural conditions. It is, however, much more difficult to 

 extract the rubber from irrigated plants on account of the increased wood 

 production. 



Reproduction of Guayule is easily accomplished by seeds which are 

 produced rather abundantly or by cuttings provided they contain some 

 of the tissue from the root zone of the plant. Plants thus produced are 

 rapid growing and usually flower in a short time as compared with seed- 

 lings. The crop is usually harvested by pulling, but the rapid reproduc- 

 tion of new plants by the old roots make it advisable to cut off the plants 

 in such a way as to leave some of the larger roots in the ground. Under 

 natural conditions it is estimated that a plant of two pounds weight can 

 be produced from seed in not less than fifteen years, while from the roots 

 of old plants a period of but nine years is required. A fifteen-year 

 rotation is advised and it is suggested that other plants which may be 

 considered as weeds should be removed to decrease competition. Har- 

 vest should take place in the summer, if the season is a normal one. 



As pointed out by the author the time available for the field work 

 upon which this study is based was limited and this is doubtless the 

 principal cause of the lack of data on environment and cultivation of the 

 Guayule plant. All the data available were used however and the work 

 is valuable as a contribution to many different lines. Histology and 

 anatomy are the most thoroughly treated and this constitutes the most 

 valuable part of the publication. The work suffers somewhat from the 

 lack of suitable chapter summaries, and a final summary would greatly 

 have increased the usefulness of the publication. — H. L. Shantz. 



