SPECIAL STRUCTURAL FEATURES. 35 



chlorophyll band of the stems also palisade — it was freciuently 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 larg-e 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 cntoryi. liplicdra aiifisvp/iililica , Ka'bcr- 

 linia spinosa; although different in certain regards the outer part of the 

 chlorenchyma of Cereiis may also be said t(j be palisade. On the other 

 hand, the following plants have a pronounced leaf-surface and the chloren- 

 chyma is spongy tissue: Condalia spathiilaca, Covillea fridriifafa, Fonquicria 

 splendens, Parkiusonia aculcata, P. torrcyana, Salix nigra, Sanibucus incxi- 

 cana. The leaves of K'raiiiiria cainscriis 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 parryi 

 has a larg-e leaf-surface and the leaves are not unlike those of Fouqiiieria; 

 like Krameria, the leaves are usually absent during dry times. The chloro- 

 phyll band is palisade. Parkiusonia 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 fairlv 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 gfiven above. The relations have 

 to do especially with the depth of the chlorophyll band, the presence or the 

 absence of pig-ment 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, e. g., those about 1 cm. in diameter, 

 there is much constancy in this regard. Aster spinosns, Baccharis enioryi, 

 and Kramer ia canes cens, 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 paHsade 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. 



