Occurrence of Foliage-lcavcs in liiisciis audrogynus. 135 



to the twisting of the organ upon itself as the primary 

 physiological object, I would suggest that such twisting 

 may be an arrangement whereby water (as rain) falling 

 upon the exposed surface would, instead of lodging in the 

 axil of the organ, either be thrown off before reaching the 

 base, or be conducted to the under surface of the base, 

 thence to run down the stem to the root. 



In the cladodineous plants above mentioned, we have 

 an interesting morphological series : from Myrsipliyllum, 

 with stomata on the morphological under side of the 

 cladode, which is not twisted ; through Buscus aculeatus^ 

 with stomata equally distributed on both sides of the 

 cladode, which is twisted a quarter-turn, so as to stand 

 vertically ; to Buscus androgynus and R. racemosus, where 

 the stomata are on the morphological upper side of the 

 cladode, which is twisted a half-turn, so as to become 

 inverted-horizontal. An evolutionist might imagine a 

 gradually progressive shifting of the stomata from the 

 morphological lower to the morphological upper side, the 

 stomata tending in the first place to become distributed 

 equally on both sides, and ultimately to be accumulated — 

 so to speak — on the morphological upper side, and that 

 such shifting of the stomata was accompanied, ^aW^assw, 

 by a progressive twisting of the organ, first into the vertical 

 position, and then into the inverted-horizontal. The diffi- 

 culty here, however, is to conceive of any external condi- 

 tions sufficient, under natural selection, to effect, in a 

 terrestrial plant, the fixation, from time to time, of small 

 variations in stomatic distribution, leading ultimately to 

 the accumulation of the stomata on the morphological 

 upper side. In an aquatic plant with floating leaves, it is 

 easy enough to imagine such a shifting of the stomata to 

 the morphological upper side ; since it is evident that the 

 smaller the number of stomata in contact with the water 

 the better the plant would fare in the struggle for exist- 

 ence, and therefore that the variations in stomatic distri- 

 bution that would be fixed by natural selection would all 

 be in one direction, viz., towards the accumulation of the 

 stomata on the upper side. But, as regards terrestrial 

 plants, the advantage of any small variations in stomatic 

 distribution can scarcely be appreciable, and seem very 



