4 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IX DESERT PLANTS. 



seasons, Covillca tridcntata (plate 1, b) and CcUis pallida (plate 2,b), the 

 former stows on the mesa and the latter on the slopes of the mountain. 

 Also what is outwardly and palpably the most extreme type of xerophyte, 

 Kirbcrlinia spinosa (plate 3, a) having leaves only when in the seedling- 

 stage, which is provided with palisade chlorenchyma, a very heav\' 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 distinguish 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 



