146 VEGETABLE PHYSIOLOGY 
are in the spongy tissue which occupies the lower half of 
the thickness of the leaf (fig. 86). The guard-cells of the 
stomata, however, always contain them. The green cortex 
of young stems and twigs also exhibits them. In such 
plants as the Caswarinas and the Hquisetums (fig. 87), in 
Fic. 86.—TRaNSVERSE SECTION oF PORTION OF THE BLADE OF THE 
Lear or Beta. 
cu, cuticle ; ep, epidermis; P.pa, palisade tissue ; S.pa, spongy tissue ; 
v.b, vascular bundle; sé, stoma; ¢.s, intercellular space, 
which the leaves are rudimentary, definite longitudinal 
bands of cells in the young stems contain them. 
The structure of such a chloroplast as is characteristic 
of one of the higher plants has not been very completely 
investigated. There is undoubtedly a protoplasmic basis 
with which the colouring matter is in some way associated. 
