246 REV.-G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
plants; as, for instance, in the dorsi-ventral structure of many 
parts of plants. In Bryopsis the reversal of the plant brings 
about a corresponding internal organic transformation. 
M. Warming notes the same feature in the leaves of plants 
growing in the arid campos of Lagoa Santa. He says *:—“ La 
direction des feuilles accuse également la sécheresse du climat ; 
beaucoup d’entre elles ont habituellement une direction verticale 
ou sont au moins trés relevées, de manicre a n’étre frappées par 
les rayons solaires que sous un angle aigu. Certaines espéces 
ont des feuilles trés réduites et quelques-unes sont aphylles ; 
elles appartiennent 4 des familles trés différentes.” The reader 
will perceive that this description would apply equally well 
to many desert plants of the northern hemisphere. 
Applying the preceding observations to desert plants :—If a 
leaf be small, narrow, and moreover assume a more or less 
vertical position, as is so generally the case, so that it is illumi- 
nated nearly equally on both sides, we should expect to find on 
a priori grounds that it would have palisade-tissue on both sides. 
Such is precisely the case with innumerable desert plants. The 
only and indeed relatively rare exceptions are in the leaves of 
such plants as develop their foliage during the rainy season, as 
annuals, or in “ Nile Valley” plants which happen to secure a 
place in the borders of the desert. Such have a more or less 
characteristic spongy parenchyma on the underside ;_ while 
transitions from this to true palisadic tissue are easy to be found. 
The typical chlorophyll-tissue in leaves of true desert plants is 
therefore palisadic on both sides, the cells being arranged in 
from one to four or even five superposed rows. These sometimes 
meet in the middle (Zizyphus Spina-Christi), in others there is 
a central layer of short rounded cells (Cassia obovata), the usual 
lax merenchyma being entirely wanting. 
In addition to the typical palisadic cells at right angles to the 
surface, large cells palisadic in shape may be arranged in 
cylinders around the fibro-vascular bundles. This is particularly 
well seen in grasses; but within this cylinder is a second, of 
chlorophyllous cells, which are quadrate and short in form. This 
peculiar arrangement of a double cylinder of green cells is not 
confined to grasses, but occurs in exogens, as Tribulus alatus, 
Atriplec Halimus, &.; so that in all cases the fibro-vascular 
cords are densely imbedded in chlorophyllous tissue. 
* Op. cit. p. 155 [supra, p. 218]. 
