8 TRANSPIRATION IN A DESERT PERENNIAL. 
occur on one side only, stomata on both sides, and the intercellularspacesare 
fewer and smaller than onthe plant kept in the green-house all its life, but are 
larger and morefrequentthanontheonefromthetree. Thesectionfrom the 
twig of the tree (D, fig. 1) shows epidermal cells with heavily thickened walls, 
from three to four subepidermal cells with walls very heavily thickened, sto- 
mata sunken into the first andsecond layers of subepidermal cells, and guard 
cells of thestomata covered withaheavy layerof cuticle. The sectionfroma 
Fra. 1.—Camera lucida drawings of leaf and stem sections of Parkin- 
sonia microphylla. 
Cross-section of leaflet from adult tree. __ 
Cross-section of leaflet from plant grown in green-house for a year and then 
placed in the open for 6 weeks. 
Cross-section of leaflet from plant kept in green-house all of its life. 
Tangential section of epidermis of twig from adult tree. 
Tangential section of twig from hot-house-grown plant. 
Cross-section of stoma from rachis of adult tree. 
. Surface view ot stoma from leaflet of adult tree. 
. Stomata from leaflets showing various apertures. 
RARER we 
twig of a green-house-grown plant (Z, fig. 1) shows epidermal cells thickened , 
but there arenosubepidermal cellsand the intercellular spaces are larger and 
more frequent than in thesectionfromthetree. Anexamination of many sec- 
tions showed that the stomata of the rachis are sunken slightly (F, fig. 1)— 
that is, that they appear alwaysnear the base of the epidermal cells, that the 
stomata of the leaflet itself sometimes appear near the bottom of this la er, 
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. 
