4 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 
seasons, Covillea tridentata (plate 1,B) and Celtis pallida (plate 2,8), the 
former grows on the mesa and the latter on the slopes of the mountain. 
Also what is outwardly and palpably the most extreme type of xerophyte, 
Keberlinia spinosa (plate 3, 4) having leaves only when in the seedling 
stage, which is provided with palisade chlorenchyma, a very heavy epider- 
mis, and with deeply sunken stomata, appears most frequently, perhaps, in 
places where the soil is quite deep. In other words, this form avoids the 
driest situations. Other forms which are leafless in dry times and there- 
fore the most of the year, as Baccharis emoryi (and perhaps Aster spinosus 
should be included, although it has annual subaerial parts), and have xero- 
phytic structure, are to be found only along the river-beds or where the water 
conditions are most favorable. Cacti, however, are usually found in dry sit- 
uations. This is probably associated with their habit of treasuring the scant 
amount of water as it comes to them from the rains, in place of depending 
on subirrigation, as in the other forms given. Prosopis, which has a con- 
stant as well as abundant water-supply, forms and sheds its leaves with the 
advent and passing of the seasons in a manner usually and perhaps always 
quite independent of the time or the amount of the rainfall. Certain of the 
more gross characters of these desert plants are thus scarcely to be attrib- 
uted to the molding influences of the environment; it will doubtless be 
necessary to take into consideration the peculiar history of each plant, its 
gradual modification from its remote mesophytic ancestor, before habits 
and structure are satisfactorily related. 
As is well known, a leading feature of the morphology of desert perenni- 
als is the reduction of the transpiring surface. Plants may be wholly with- 
out leaves, or leaves may be present during early growth or during favor- 
able seasons only, or if leaves are a feature they may be much reduced in 
size (plate 4). In the former instances the twigs and the branches assume 
the functions of leaves; in the last case it will be shown in this paper that 
the same is also true when leaves are present but reduced in size or present 
during favoring seasons only. 
Among other characters which distinSuish the leaves of xerophytes is the- 
palisade nature of at least the subepidermal portion of the chlorenchyma. 
That is, the chlorophyll tissues of the leaf are to a greater or less extent 
~ composed of cells whose long axes are placed at right angles to the surface 
of the leaf. It is of interest, therefore, to learn how far the structure 
characteristic of the leaves is found in such stems as exercise the function 
of leaves. 
To anticipate one of the findings of this paper, in plants whose transpiring 
surface is most perfectly reduced the chlorenchyma of the stem is in certain 
regards very like that in the leaf of the same species; but in those with a 
more or less pronounced leaf-surface the chlorenchyma of the stem is unlike 
that of the leaf. In the former case the stem structure is palisade; in the 
