LEAF-BLADDERS IN EICHHORNIA SPECIOSA. 223 



Discussion : — From the whole of the evidence available it seems 

 reasonable to attribute the appearance and disappearance of the 

 bladder to changes in the medium which influence the absorption of 

 water in each case. Firstly, there are plants swimming in deep water 

 which is freely exposed to light and air and in which the maximum 

 absorption is facilitated by a low concentration. The primary 

 effect of such a decrease in the osmotic concentration is, as 

 Livingston (VIII) points out, " to add water to the organism whereas 

 an increase in the concentration has a drying effect ". Secondly, 

 there are plants in deep water crowded together so closely that light 

 does not penetrate into it and the medium is also relatively lower 

 in temperature. Thus a difference of 1° C was noticed in the same 

 pond at noon on a sunny day up to 18 inches depth in the midst of 

 the two kinds of plants. The roots in' such plants are also 

 covered by the mass of putrefying remains of the older leaves 

 and roots which necessarily produce carbondioxide and various toxins 

 that check rapid absorption of water. Thirdly, when plants are 

 fixed to mud the roots become subject to (l) deficiency of oxygen, (2) 

 coldness of the soil, and (3) higher concentration of water. The 

 greater absorption of water or its check is thus due not to any single 

 factor but to the co-operation of numerous factors which determine 

 the water content of the plant. This water content may be best defined 

 as " a function of the relation that has previously obtained between 

 the rates of water entrance and of water exit," " it being immaterial 

 whether it becomes low through high rates of water loss, or through 

 low rates of water intake" (Livingston, Ed. of Palladin's Plant Phy- 

 siology), (9). It also appears possible that the very early development 

 of axillary shoots resulting in the formation of the sympodium depriv- 

 es the original shoots of water and an elongation of the stalk beneath 

 the bladder is rendered impossible. This is well seen in the absence 

 of swellings in the later formed leaves and in the actual elongation 

 of the stalk when the axillary shoots are slow of development. 



The question then arises about the real nature of the plant. 

 From the facts stated above it will be clear that the distension of 

 the stalk is not so much an adaptation as a self-adjustment to 

 the medium which aquatics in particular display owing to the 

 extreme plasticity of organs characteristic of them. Goebel (3) who 

 paid some attention to this question in his Pflanzen biologische 

 Schildertingen confesses the inadequacy of the explanation on biologi- 

 cal grounds, for the bladders are formed above water and the leaf 

 floats with or without it. He noticed the disappearance of the 

 swelling in the later formed leaves though the illustration (4) given in 

 support of this actually resembles one of the transitional stages in 

 which the leaves undergo a partial swelling. A study of the life 



17GC-29 



