GUAVrLF. Wn M AKIOI.A. 201 



delicately frilled, making a most striking ticNiation from the 



main type, between which numerous intermediates are to be 

 found. 



The more prominent structures by which these annuals 



are fitted for life in the desert are not to be looked for in the 

 shoots or lea\-es, but in the seeds and their po\\ers of endur- 

 ance. Seeds are ripened and thrown on the ground in March 

 and April. The surface layers of the soil reach a temperature 

 of o\er 1 <)() F. during the summer months, the rains c<»me 

 and soak both the soil and seeds, but still nn acti\ity is shown, 

 and the experimentalist who attempts to use these plants dur- 

 ing the summer will find that he might as well have sown so 

 many pebbles in his pans. The summer cools into the autumn, 

 and cooler nights come, followed by the winter rains of De- 

 cember; then and not until then, do these refractory seeds 

 begin to show signs of life. Two features are possibly in- 

 volved in this delayed germination. One is that the seeds 

 need a certain length of time for the carrying out of slow 

 changes toward maturity, which take place during the so- 

 called resting season and which require a period of deter- 

 minate length not to be shortened. Secondly, it is quite pos- 

 sible that in some species the baking summer heats, the moist 

 soil, the cool nights of autumn, and the rains are a series of 

 stimuli which must follow each other in turn and act for a 

 length of time before the seedling emerges from its protecting 

 coat'3. Favor is lent to this \-iew by the fact that in some species 

 germination may be induced earlier by simulating the sum- 

 mer heats -Mid. the winter coolness by the use of the oven and 

 the refrigerator. 



METHODS OF VKGK TAriX K REPRODL CTION 1\ 

 GlJAYl LI-: AND MARIOF.A. 



By Ik.wcis ]".. Li.ovi). 



Guayule {Pdrl/u'iiiinii tir'^ciildliiiii (jrav) and mariola 

 (P. iii'.uuiiini H. B. K.) are two closely related shrubby 

 species of the genus Pdrl/wuiiaii , both occupying similar hab- 

 itats though in \ar\ing iMtios in the desert regions of the nirsn 



