226 REV. G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
chymatous cords at the angles were quite as much developed as 
in normally grown stems. 
Moreover, all these well-developed mechanical elements of 
consolidation in the Dioscorea were in a greater relative pro- 
portion, in consequence of the considerable reduction which the 
parenchymatous tissues had undergone. 
The leaves were small and undifferentiated, the parenchyma 
between the veins being much arrested; the stomata were un- 
developed and the palisade and lax central parenchyma all alike 
and unformed in character, except that the two of four larger 
layers in the centre had no chlorophyll. 
From the above brief ‘epitome of M. Duchartre’s experiment 
it will be seen that, so far as the small amount of parenchyma 
and the great density of the mechanical elements of the fibro- 
vascular tissues are concerned, we have an exact parallel between 
desert plants and this Dioscorea grown without water in the 
dark. Hence this experiment, together with those referred to 
above, will suffice to completely justify the conclusion that 
the indurated character of the mechanical system and also the 
spinescent features of so many desert plants are simply the imme- 
diate results of the effects of the comparatively waterless cha- 
racter of the environment. Lastly, it must be borne in mind that 
the spinescent character is hereditary, and although the rigidity 
begins to break down under cultivation in a moist climate and a 
good soil, the spines may still be formed but become gradually 
modified and finally disappear, reverting to leafy branches, as has 
occurred with cultivated pears, plums, and Ononis; or back into 
leaves, as in Berberis. 
That “xerophilous ” peculiarities of plants are not only cor- 
related with but are actually caused by the arid conditions of 
their environment is the conclusion of A. F. W. Schimper *, who 
has lately studied the flora of Java. He finds that the plants of 
the shores, alpine plants, those of the solfataras, and epiphytes, 
although very different from each other in a classificatory sense, 
all present common characters in being strictly “ xerophilous.” 
In all, the leaves are small and thick, there is a strong cuticle, 
* “Ueber Schutzmittel des Laubes gegen Transpiration, besonders in der 
Flora Java’s” (Mittheilungen aus den Sitzungsberichten der konigl. preuss. 
Akademie der Wissenschaften zu Ber n, 1890, Heft vii. p. 1045). See also 
Rey. gén. de Bot. 1892, p. 364. 
