36 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 



enchyma in the stem. With its extremely thick cuticle, 80 p-, Kceberlinia 

 is a marked exception to this condition. Contrast the depth of the band as 

 given for these species with that of the stems of Ccltis fiallida, Covillca triden- 

 tata, Parkinsonia representing the leafy class of parennials. The figures 

 are 96 p-, 48 M, and 83 ^, respectively. All of these forms have spongy 

 chlorenchyma in the stem. 



There is also fair consonance between the kind of chlorenchyma and the 

 kind of tissue exterior to it. In Aster, Bacc/iaris, A'ramcria, Ktcberlinia, 

 Cereus plants with a reduced leaf-surface or with no leaves the chlorophyll 

 band extends to the epidermis. Such is the condition to be met, not alone 

 in young, but also in old stems of these plants, which, however, is not the 

 final condition in most of them. In the type of plants with larger leaves, 

 or larger leaf-surface, on the other hand, there is frequently when young, 

 and always when old, some sort of tissue, in addition to the epidermis, which 

 serves as a covering to the chlorophyll band. This may be hypodermal 

 tissue, cork in most plants, or in Olneva a curious proliferation of the epi- 

 dermal cells in the young branches by which a many-layered epidermis is 

 organized. It will be recalled that in the former class of perennials the 

 chlorenchyma is palisade and in the latter spongy tissue. 



In yet another structural characteristic the two classes of plants are dis- 

 tinguished. With certain exceptions plants with rudimentary leaves or none 

 have no pigment in the epidermis, i.e., the parts exterior to the chlorophyll 

 band are colorless. Biit in Ccltis, Condalia, Olneya, and Prosopis all forms 

 possessing a pronounced leaf -surface, either the epidermis or the hypoderm, 

 or again the periderm, is provided with a pigment which is usually of a 

 dark-red color. In these forms the chlorenchyma is of spongy tissue. An 

 exception to this is found in Parkinsonia, which has no pigmented tissues 

 exterior to the chlorophyll band. 



The coincidences which have been repeatedly observed of spongy chlor- 

 enchyma and pigmented exterior tissues lead to the belief that the relation 

 between the two is something more than a chance association. This is 

 strengthened by the further observation that in palisade chlorenchyma no 

 such exterior pigmented cells are usually to be found. 



Although no hard and fast rule can be given, it appears that perennials 

 which have a reduced leaf-surface present the following characteristics 

 regarding the chlorophyll band of the stem: Its structure is wholly or at 

 least in part palisade; it usually lies close to the surface, and the tissues 

 exterior to it usually do not contain pigment. While, on the other hand, 

 perennials with a pronounced leaf-surface possess a chlorophyll band of 

 spongy tissue, there is usually some kind of tissue in addition to the epider- 

 mis between it and the surface, and the exterior tissue usually contains 

 pigment. 



