Reproduction. 75 



than in the shade. Thus, in February 1908, small seedlings with i to 5 

 foliage leaves could be found beneath the shade of an occasional larger 

 guayule plant, but in a precarious condition, some dead, others moribund, 

 and plainly the survivors of the crop of seedlings of late in 1907, the chief 

 part of which had succumbed to the very severe conditions already noted 

 as having prevailed at Cedros at that time. As bearing upon this ques- 

 tion, we may note the meager occurrence of Opuntia leptocaulis in south- 

 ern Arizona, where it is scarcely to be found except protected by some 

 plant, while it grows in the open in great abundance in Zacatecas. It 

 appears evident that in Arizona the conditions for its persistence, except 

 when it is more or less protected by other plants, are too severe. No such 

 relation has been observed in Zacatecas, and it would seem that the cli- 

 matic conditions there are distinctly more favorable for this plant. 



It would therefore appear safe, if desirable, to clear guayule fields 

 of the major part of other vegetation. An occasional year may be ex- 

 pected when the rate of germination will go far toward producing a good 

 stand of young plants. Those already growing will offer protection to 

 the younger brood, and the larger area available for guayule plants will 

 in part compensate for the loss of shade given by other vegetation. It 

 would not be advisable, however, to remove the occasional palma saman- 

 doca (Samuella carnerosd) , which produces fiber, or the large barrel cacti 

 (" bisnaga burra " and " bisnaga colorada ") , as they are heavy plants and 

 neither spread with appreciable rapidity nor occupy more than a negli- 

 gible fraction of the ground (plate i, fig. A). This principle of practice 

 is, however, in the nature of a compromise, and rests upon an estimated 

 balance of circumstances. A more correct estimate of probabilities could 

 be based only upon longer observation under experimental conditions. 



RATE OF REPRODUCTION AND OF GROWTH. 

 RATE OF GROWTH DURING GERMINATION. 



This period may be divided into a period of tissue expansion and 

 one of induration. At the close of expansion, which begins in about a 

 week's time after sufficient rain, and occupies a second week, the seedling 

 is tender, the hypocotyl white and translucent, and the cotyledons green 

 (fig. 8). The cuticle then thickens, and red color is developed in the 

 epidermis of the hypocotyl and under surface of the cotyledons, while the 

 latter become darker green and more indurated. This occupies a third 

 week, when, if no untoward circumstance interferes, the first foliage 

 leaves develop. Even under the best of conditions this period of three 

 weeks will scarcely be shortened. 



The further seedling development is a direct function, other things 

 being equal, of the rainfall, the maximum potentiality, it may safely be 

 said, never being exerted by field plants. This apparently extremest 

 limit of growth for a seedling was reached by one of two particular indi- 

 viduals under cultivation, and constantly supplied with an abundance 

 of water. The height of this plant when the rhythm-limit was reached, 

 as indicated by cessation of growth, was 25 cm., and it had a spread of 

 22 cm. It was a fully-developed specimen, in which each branch reached 



