THE STEM AND THE LEAF 57 
all the year round, and in these photosynthesis is carried on 
by the green layer of the cortex beneath the epidermis of 
the stem. 
In many dry 
countries there 
are switch plants, 
which either bear 
a few little leaves 
during a small 
part of the year 
or are entirely 
leafless, as is 
shown in figure 
387. Like the cacti, 
such plants, and 
others with flat- 
tened stems and 
little leaf sur- 
face (fig. 38), do 
the greater part 
or all of their 
photosynthetic 
food-making in 
the green cells 
of the cortex of 
the stem. 
Many of our 
common forest 
trees and shrubs 
do food-making 
in the green layer 
of the bark of 
the young twigs, 
until this is shut 
away from the 
light by layers 
Fic. 38. Branches of Muehlenbeckia, a plant with 
scanty leaf surface and flattened stems which do 
most of the photosynthetic work of the plant 
J, flowers; J, leaves; », stems. One half natural size 
