INFLUENCE OF THE MEDIUM. 501 



ground was well moistened and warmed; but this they could obtain in abundance 

 from the saturated clay soil containing the humus. 



It has been already stated that the ground water is less favourable for vegetation 

 than rain and dew on account of its paucity of carbonic acid. But the moistening 

 of the ground by water which wells up from below brings other evils in its train. 

 By this means the soil is over-saturated for a long time, a condition which the roots 

 of most land-plants will not tolerate. When it remains stationary for a long while 

 potassium and sodium salts, and, under certain conditions, humous acids pass into it 

 from the wet earth in quantities anything but advantageous to the plants. Vege- 

 tation, therefore, exhibits a scanty growth in places where the ground water 

 influences the stratum of soil penetrated by rodts, and it usually consists of 

 comparatively few species. 



In low-lying regions, where the ground water rises to the surface, we have the 

 formation of lakes and ponds with variable water-level. Sometimes the plants 

 growing in such places are quite submerged, while at other times their stem 

 and leaves are above water. Land plants do not take kindly to this. Most of them 

 cannot survive very long immersion; they become suffocated, die, and decompose 

 under water in a few days. Only a few species have the remarkable power of 

 growing equally well below or above water, and these are, of course, extremely 

 interesting on account of their form. In accordance with the great contrast 

 presented by the external conditions of life to which these species are temporarily 

 exposed we have a fundamental change both in their outward appearance and in 

 the internal structure of their several organs. In order that the stem and leaves 

 should be held in the best position by the flowing water, the mechanical tissue in 

 ;submerged varieties of these species is much reduced (see vol. i. pp. 424 and 665). 

 They are also devoid of the contrivances which usually regulate transpiration, since 

 no evaporation occurs under water. Stems grown under water consequently appear 

 limp and flaccid when taken out of it; their leaves, when compared with those 

 growing in the air, are much weaker and more delicate. They have no gloss, but 

 are brighter green in colour, and in the air they collapse and dry up in a very 

 ■short time. A vertical section through the leaf shows that the number of cells 

 between the upper and lower epidermis is much reduced, and that the cells are 

 shortened in a direction perpendicular to the leaf surface. The foliage-leaves of 

 Veronica Beccahunga, when grown under water, are hardly one-third as thick as 

 those grown in the air, and between the upper and lower epidermis there are only 

 4-5 layers of short cells, while in corresponding leaves of aerial plants there are 

 10-12 cell-layers and a distinct division into palisade and spongy parenchyma (see 

 vol. i. p. 279). The shape of the leaf is also much changed under water. In 

 Veronica Beccahunga the difference in aerial and submerged leaves is very slight, 

 consisting only in the shortening of the petiole and in the marginal teeth becoming 

 less marked. In Veronica Anagallis, likewise, the alteration in shape is incon- 

 ■siderable, but in many others it is very noticeable, and we shall return to it when 

 speaking of the influence of light. 



