SPECIAL STRUCTURAL FEATURES. 35 
chlorophyll band of the stems also palisade—it was frequently spongy 
tissue.* There is a relation between the structure of the chlorenchyma of 
the stem and the foliar habit of the plant which holds in all well-marked 
cases and which comparative studies on the forms and their relatives may 
show to be valid in cases now not so clear. The relation may be stated 
thus: In perennials with no leaves, or with rudimentary leaves, the chloro- 
phyll band of the cortex is structurally palisade tissue. On the other hand, 
perennials with relatively large leaves, or a large leaf-surface, have chloro- 
phyll bands of spongy tissue. The following-named plants either have no 
leaves at any time of the year (in mature stage) or the leaves are clearly 
of a rudimentary nature and the chlorenchyma of the cortex is uniformly 
palisade: Aster spinosus, Baccharis emoryi, Ephedra antisyphilitica, Keber- 
linia spinosa, although different in certain regards the outer part of the 
chlorenchyma of Cereus may also be said to be palisade. On the other 
hand, the following plants have a pronounced leaf-surface and the chloren- 
chyma is spongy tissue: Condalia spathulaca, Covillea tridentata, Fouquieria 
splendens, Parkinsonia aculeata, P. torreyana, Salix nigra, Sambucus mext- 
cana. The leaves of Kramcria canescens should probably be considered 
rudimentary, although of fairly large size; during the driest seasons the plant 
is leafless. The chlorenchyma of the stem is palisade. Zizyphus parryt 
has a large leaf-surface and the leaves are not unlike those of Fouguieria, 
like Krameria, the leaves are usually absent during dry times. The chloro- 
phyll band is, palisade, Parkinsonia microphylla, as the specific name 
indicates, has very small leaves, so small that their presence is hardly noted, 
and yet the chlorenchyma of the stem is of spongy tissue. It should be 
said also that the leaves of P. microphylla, as well as those of the most leafy 
forms, fall away with the advent of dry seasons. 
Whatever may be the significance of this variation in structure of chlor- 
enchyma of stems of perennials, there appears to be a fairly uniform relation 
between it and the character of the tissues exterior to the band. The 
exceptions to this relation are at least no more striking than the exceptions 
to the relation of structure and leaf-habit given above. The relations have 
to do especially with the depth of the chlorophyll band, the presence or the 
absence of pigment in the exterior tissues, and perhaps also with the 
presence or absence of trichomes. 
As a rule, the depth of the chlorophyll band may vary with the age of 
the stem; however, in young stems, ¢. g., those about 1 cm. in diameter, 
there is much constancy in this regard. Aster spinosus, Baccharis emoryi, 
and Krameria canescens, all of which must be considered plants with a 
reduced transpiring surface, have the following depths of the chlorophyll 
band: 19.2 », 16 #, 18.8 #, respectively. These forms have palisade chlor- 
*The leaves of Opuntia versicolor, and perhaps of other opuntias, do not appear to 
have palisade tissue, although palisade-like tissue is to be found in the permanent parts, 
namely, the stems. Compare fig. 14, Biological Relations of Certain Cacti, W. A. Cannon, 
American Naturalist, 1906, 40 : 27. 
