8 



TRANSPIRATION IN A DESERT PERENNIAL. 



occur on one side only, stomata on both sides, and the intercellular spaces are 

 fewer and smaller than on the plant kept in the green-house all its life, but are 

 larger and more frequent than on the one from the tree. The section from the 

 twig of the tree (D, fig. l) shows epidermal cells with heavily thickened walls, 

 from three to four subepidermal cells with walls very heavily thickened, sto- 

 mata sunken into the first and second layers of subepidermal cells, and guard 

 cells of the stomata covered with a heavy layer of cuticle. The section from a 



r(J 



FIG. 1. Camera lucida drawings of leaf and stem sections of Parkin- 

 sonia microphylla. 



A. Cross-section of leaflet from adult tree. 



B. Cross-section of leaflet from plant grown in green-house for a year and then 



placed in the open for 6 weeks. 



C. Cross-section of leaflet from plant kept in green-house all of its life. 



D. Tangential section of epidermis of twig from adult tree. 



E. Tangential section of twig from hot-house-grown plant. 



F. Cross-section of stoma from rachis of adult tree. 

 O. Surface view ol stoma from leaflet of adult tree. 

 H. Stomata from leaflets showing various apertures. 



twig of a green-house-grown plant (E, fig. l) shows epidermal cells thickened, 

 but there are no subepidermal cells and the intercellular spaces are larger and 

 more frequent than in the section from the tree. An examination of many sec- 

 tions showed that the stomata of the rachis are sunken slightly (F, fig. l) 

 that is, that they appear always near the base of the epidermal cells, that the 

 stomata of the leaflet itself sometimes appear near the bottom of this layer, 

 but that the usual place is near the top of the layer, and that the stomata of 

 the branches are always sunken below the layer of epidermal cells. 



