172 



PLANT WORLD. 



masses of humus material due to the decay of preceding gen- 

 erations of non-storing types. In all locations the non-storing 

 types with rosettes for the external retention of water are 

 found in places where there is no water-retaining substratum, 

 their supply of storage water being caught as it falls. 



A comparison of the rates of transpiration in Stelis 

 when supplied with water and when subjected to drought by 

 the removal of the leaves, indicates that it is able to cut 

 down its water loss to such a low point that its reserve will 

 suffice to meet the demands of transpiration for a period 

 something short of 50 days. The velamen is not well devel- 

 oped in the roots of the Stelis, which are slender as compared 

 with the roots of most epiphytic orchids, and are invariably 

 to be sought beneath mosses, bark or debris, while roots 

 with velamen are just as often found creeping over naked 

 limbs or hanging free. The external supply of water is, 

 then, of almost as great importance to the plant as is its 

 own store. When external water is abundant transpiration 

 goes on actively, and in amount not greatly different from the 

 current absorption day by day. When the external supply is 

 cut oft the transpiration rate is at once lowered and the 

 plant enters upon a period of reduced capacity for vegetative 

 activity. 



Desert Botanical Laboratory, Tucson, Arizona. 





SOME FEATURES OF THE ANATOMY OF 



GUAYULE {PARTHENIVM ARGEN- 



TATUM GRAY). 



By Francis E. Lloyd. 



There has recently appeared a paper by Dr. H. Ross 

 dealing with the anatomy of the Mexican guayule, a plant 



*])er anatoinische B;iu <ler Mt'xilcaniscliPii Kautscliukpllanze "Oua,\-\ilp", 

 raithcniiim ai-ticiitatnm Cray. Ber. d. I5(jt. OwscU. 26a; IMS-lid:^. \\>.. I'.MIS. 



* 



