Reproduction. 73 



Countless numbers of seeds are therefore strewn upon the ground, and 

 indeed the new plants of guayule which spring up on these stack-grounds 

 sometimes afford valuable data on the rate of growth, although decep- 

 tive notions as to the numbers of plants which may be expected are some- 

 times acquired. The conditions of a stack-ground are, at Camacho at 

 least, a rather severe test, as it lies out in the open, dry plain, exposed to 

 full sunlight. At the same time, the surface of the soil is mulched by the 

 debris of broken-off guayule twigs, and thus the conditions are amelior- 

 ated. Shipments, which had been made for a year or longer, ceased in 

 the fall of 1907, and the spot was under occasional observation for some 

 time before and from that time on, till the following September. Al- 

 though it is known that much guayule was brought in in flowering con- 

 dition and that seed must have been dropped in large quantities, the 

 conditions for germination, especially the meager rainfall, were not favor- 

 able for guayule, though the seeds of the plants in the following list 

 were found in all conditions of development in June and July of 1908: l 



Helianthus sp. 15 to 18 inches tall and many in flower. 



Amaranthus, 2 species. Plants 3 inches tall. 



Cassia ("coco"). Many mature plants in flower. 



Prosopis seedlings with the plumule well developed. 



Euphorbia of 2 species. Mats 10 inches in diameter. 



Solatium sp. 



A cucurbitaceous vine. 



Chenopodium, a species with broad deltoid leaves. 



Grasses of 4 species. 



Spheralcea, mature plants. 



In addition to these seedlings, the roots of Prosopis and Covillea, 

 which had been cut off in preparing the ground for stacking, had sent up 

 shoots from 10 to 20 inches in length. That not a single guayule plant 

 sprang up is at first surprising, not to say disconcerting, but in the light 

 of experimental evidence it becomes clear that the guayule germinates 

 only under highly favorable conditions. For some time it has a low de- 

 gree of resistance, and is in point of fact of distinctly mesophytic charac- 

 ter. It is only when due regard to this is had that the maximum rate of 

 germination may be expected under cultural conditions. 



HABITATS OP SEEDLINGS. 



The particular preference of the guayule for certain germination 

 habitats is of importance in its bearing on the effect of clearing land of 

 other plants. It has been repeatedly observed by the investigators at 

 the Desert Botanical Laboratory, and by myself in Zacatecas, that there 

 are usually to be found many more plants of smaller size growing in the 

 partial shade of shrubs than elsewhere, and it is to the protective effect 

 of this shade that the many curious juxtapositions of perennial plants 

 may be referred. An example of this is the frequently seen saguaro 

 (Carnegeia gigantea) , standing in a position indicating that it germinated 

 in the shade of a palo verde (Parkinsonia microphylld) or some other 

 shrubby species. 



As regards the guayule, Endlich (1905) speaks of "the large numbers 

 of young plants sometimes found surrounding the older trees * * * in the 



1 Every annual had disappeared by April 1909. 



