AND OF SALINITY. 265 



produced long shoots. The stem was more slender than 

 usual, but excessively rigid, owing to the reduction of 

 the parenchymatous tissues, and the predominance of 

 the elements of consolidation. The leaves were small 

 and undifferentiated, and the stomata undeveloped. 



Many other instances showing the relations between 

 floral structures and arid surroundings have been col- 

 lected by Henslow in his book on the " Origin of Plant 

 Structures," where the subject is dealt with in extenso. 



The effects of a very great increase in the humidity 

 of the surroundings, such as is experienced by plants 

 which actually live in water, lie, as might be expected, 

 in a very different direction. That the peculiar char- 

 acters of aquatic plants are in considerable measure the 

 direct effects of their peculiar environment, is proved 

 by the fact that plants, normally terrestrial, often de- 

 velop such characters when grown in water. Thus 

 Costantin found that under such conditions a diminu- 

 tion in the number of the vessels of the fibro-vascular 

 system of the stem invariably occurred. For instance, 

 in Vicia sativa (Vetch) the middle of the stem of the 

 aquatic form of the plant had only 38 vessels, whilst the 

 aerial form had 47. In Ricinus communis (Castor-oil 

 plant) the aquatic form had 10, as against 21; in Faba 

 vulgaris it had 2 at the sides and 15 at the angles, as 

 against respectively 5 and 36. The pseudo-aquatic 

 forms thus tended towards true aquatic plants, in which 

 the fibro-vascular system is always more or less de- 

 generate. Again, Costantin found that there was an 

 increase in the lacunae, when stems normally aerial are 

 kept submerged, just as the aquatic form of amphibious 

 plants is found to have more of such lacunae than the 



