Chap. I] 



WATER 



2 3 



Many terrestrial plants growing accidentally in water exhibit only slight 

 deviations from their normal structure, for this is too firmly fixed by 

 heredity to yield, in the first generation, to new influences. Other terrestrial 

 plants are more plastic and at once undergo a series of modifications owing 

 to which their structure approaches that of true aquatic plants. Thus 

 H. Schenck found on the banks of a pond, which had overflowed, some 

 submerged specimens of Cardaminepratensis which exhibited the following 

 deviations from the normal terrestrial form. The cauline leaves, normally 

 sessile, had acquired long petioles, their segments were narrower, their 

 mesophyll was thinner and devoid of palisade-cells, their cortex was thicker 

 because their vascular bundles had been displaced towards the centre 

 (Fig. 29), the sclerenchymatous elements richly developed in the terrestrial 

 form were absent, the outer wall of the epidermis had become very thin, 

 the vessels were greatly reduced, and the intercellular spaces enlarged. 

 These modifications are to a great extent very similar to those induced 

 by water-vapour. In very damp 

 air we find lengthening of the 

 petiole, diminution in the thick- 

 ness of the cell-walls, reduction in 

 the development of vessels and 

 palisade-cells, and an increase in 

 the air-containing spaces. Only 

 two characteristics, which are 

 not very prominent, depend on 

 the liquid condition of the water : 

 these are centripetal displacement 

 of the vascular bundles and the 

 narrowing of the leaf-segments. This represents the first step towards the 

 transformation of a terrestrial plant into an aquatic plant. 



The Cardamine does not appear able to hold its own as an aquatic 

 plant. Its plasticity is not sufficient for the purpose. Other so-called 

 amphibious plants, the best known of which is Polygonum amphibium, 

 thrive equally well as aquatic or as terrestrial plants, because, owing to 

 a high degree of plasticity, they become appropriately modified for either 

 medium. 



Aquatic pJianerogams and pteridophytcs, possibly also aquatic mosses, have 

 originated from plastic terrestrial plants that possessed the faculty of estab- 

 lishing themselves as aquatic plants. Excepting the few species that have 

 remained amphibious, their persistence is eventually due to this faculty, 

 for, crowded out by the competition of terrestrial plants, they have taken 

 refuge in the water, where they have gradually become appropriately 

 modified and have lost partially, or completely, the ability to thrive 

 normally upon land. 



Fig. 29. Cardamine pratensis. Stem in trans- 

 verse section. A Terrestrial form. B Aquatic 

 form. 111 pith; rp cortical parenchyma ; mr ring 

 of mechanical tissue. Slightly magnified. After 

 H. Schenck. 



