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 Physiology), (9). It also appears possible that the very early development of axillary shoots resulting in the formation of the sympodium deprives 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 biological 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
