THE ORIGIN AND STRUCTURE OF AQUATIC FLOWERING PLANTS. 89 



wet land ; for that type of leaf is characteristic of crowded foliage, as 

 seen in thrift, pinks, &c., and it occurs in many tufted plants, as 

 Plantago laiiceolata, and especially if aquatic, as Littorellalacustris , &c. 

 Then, when the marsh became a pool, Banwicidus Lingua was sub- 

 merged, or else it bore floating leaves; for these two species put forth 

 quite different forms under these conditions, the blade of the submerged 

 leaf being oblong, broad, and blunt at the end ; while the floating form 

 is more heart-shaped and rounded, like that of the water-lily, but small. 

 Both these species are now amphibious; but that type of submerged 

 leaf is exceptional among Dicotyledons. 



The general effect of an excess of w^ater on the anatomical structure 

 of plants is degeneracij. Yet, while aquatic plants have many of their 

 organs and much of their internal tissues more or less arrested^ — i.e. as 

 compared with those of land plants — the plant thereby puts itself in 

 adaptation to a submerged existence, inasmuch as many tissues, which 

 cease to be produced, are no longer required. As a result, large hollow 

 spaces are formed between the cells for the accumulation of air, partly 

 to sustain vital functions and partly to act as floats. 



Contrasts between the Effects of Moisture and Drought. — The 

 first superficial and readily observable effect of a damp atmosphere is 

 the greater or less absence of hair on the stem and leaves. Thus, the 

 silver- weed (Potentilla anserina), if growing in a dry, sunny situation, 

 as by a roadside, has both sides of the leaf densely covered with silky 

 hairs, giving a silvery appearance, hence its name: but in a damp, 

 shady lane the upper side is mostly destitute of hair, so that it acquires 

 a dark green colour. Similarly all the species of rush (Juncus) are 

 glabrous, and grow in water or damp places; while the woodrush 

 (Luzula) is hairy. 



Different species or varieties of the same genus may show similar 

 differences. Thus the Forget-me-not (Myosotis palustris) and its ally 

 M. caespitosa, being aquatic or marsh plants, are very scantily sup- 

 plied with adpressed hairs, while M. sylvatica, a land plant which is 

 often cultivated, has numerous spreading hairs. So, too, buttercups 

 are hairy plants, but the water crowfoot is quite hairless.''' 



The Degenerative Effects of Water upon Roots. — This is well 

 seen in the germination of a seed, for the radicle is at once arrested 

 if it penetrate water, as may be observed by placing some seeds of 

 dicotyledonous plants (cress, peas, &c.) on a piece of net just touching 

 the surface of the water; as soon as the radicle penetrates the water 

 it ceases to grow, but adventitious roots then arise from the stem or 

 hypocotyl above the apex. These lateral roots formed for use in water 

 cannot be used by the plant if transferred to land. Several aquatic 

 plants are entirely rootless in a full-grown state, as the Hornwort 

 (Ceratophyllum), Bladder-wort (Utricularia), &c. 



Other characteristic features of aquatic roots are seen in their 



* If Banunculus repens happens to grow more or less by water, the hairs 

 become ad'pressed. They are peculiar in having a compound cellular base (as 

 in Boraginae) , probably for absorbing water as dew in dry situations. 



