LAKES, RIVERS, ETC. 



249 



The adaptations to aquatic conditions in 

 internal structure have been referred to. The 

 adaptations in external form or leaf arrange- 

 ment are exemplified by the finely-divided 

 or dissected foliage of Water Buttercups, 

 Water Dropwort, Water Violet, Milfoil, 

 Horn wort, Bladdervvort. The carbonic acid 

 gas in water is more abundant but less avail- 

 able, hence the dissected type of leaf, which 

 also offers less resistance to the current, and a 

 greater relative surface exposed to the light, 

 and for the absorption of oxygen. The ribbon 

 type of leaf is an adaptation to the currents, 

 and is found in Water Plantain, Arrowhead, 

 Flowering Rush, &c. 



Another type is the clustered awl-shaped 

 leaf, as in Water Lobelia, Shoreweed, Pill- 

 wort, &c. Floating leaves may be orbicular, 

 as in some types of Water Buttercup, Water 

 Lilies, Frogbit, or ribbon-shaped, as in Manna 

 Grass, Bur Reed, Bulrush. The Water Butter- 

 cups and some other types have two types of 

 leaves, adapted to either the submerged or 

 floating position. 



The Height of Water Plants. The aquatic 

 plants have different conditions to contend 

 with in regard to the height to which they 

 usually grow. A land plant, whether it be a 

 shade plant or a sun plant, obtains the neces- 

 sary light, and other factors essential for growth 

 and nutrition, more or less directly. It has no 

 great period of subterranean existence to out- 

 live after the seed has germinated before it 

 reaches the surface and the light. Nor is the 

 light variable in the character of the rays ab- 

 sorbed, or in intensity to any considerable 

 extent. 



Aquatic plants, on the other hand, have to 

 struggle upwards in the dark after germina- 

 tion, and the young shoots have to make some 

 growth before they can obtain even a moderate 

 degree of light. Hence they must attain a 

 certain height under water before their chief 

 functions can be adequately performed. This 

 tends to make the submerged types generally 

 of a uniform height in each zone, and those 

 that struggle upward to put forth floating 

 leaves also attain a more or less uniform 

 length or height. Except in the case of the 

 half-submerged types of the reed swamp, few 

 become erect either below or above water. 



The tallest plants, ranging from 3-10 ft., 

 grow in the reed swamp, and are erect or 

 nearly so. But the floating plants and sub- 

 merged types may attain a great length, 

 though the visible height above water is trivial 

 in comparison. 



The Flowering of Aquatic Plants. As in the 

 case of all moisture-loving plants, the season 

 of flowering of aquatic plants is on the whole 



late. The riparial plants, the least immersed 

 in water, flower relatively early in the year. 

 The immersion of aquatic plants in water, 

 colder than air, is responsible for the late stage 

 at which the respective thermal constants of 

 each plant are reached. It is an interesting 

 fact that a large number of marsh and aquatic- 

 plants have the leaves and stem coloured red 

 by anthocyan, which transforms the light rays 

 into heat. 



The earliest flowering aquatic or riparia! 

 types are Coltsfoot and Butterbur, and Snakes- 

 head Fritillary, which flower in March. In 

 April a small number of others first commence 

 to bloom, as Water Fennel, Scorpion (irass, 

 Crack Willow; and these, save the fir>t, are 

 riparial, or but slightly submerged, if at all. 



In May, Meadow Rue, Water C'ress, Water 

 Betony, Brooklime are in flower. The follow- 



ing are found 

 temperature 



flower in June, '.vhen tin- 

 much higher, viz.: Yellow 



Water Lily, Great Water Cress, Creeping 

 Jenny, Musk, Skullcap, Amphibious Knot- 

 grass, Yellow Flag, Sweet Flag, Duckweed, 

 Water Plantain, Flowering Rush. July is the 

 principal month for aquatic plants to flower, 

 and then one may find the blooms of the 

 White Water Lily, Great Water Chickweed, 

 Purple Loosestrife, Great Hairy Willow Herb, 

 Marsh Bedstraw, Three-lobed Butterbur, 

 Water Ragwort, Mint, Gipsywort, Frogbit, 

 Reed Mace, Bur Reed, Arrowhead, Bulrush, 

 Wood Club Rush, Reed. In August, Hemp 

 Agrimony and Fleabane first come into flower. 



The Duration of Aquatic Plants. Tin- great 

 difference between land and water plants has 

 an important effect upon the duration of the 

 latter and their mode of seeding. 



It is much more difficult, in fact, for an 

 aquatic plant to germinate, put forth aerial 

 shoots, branches, leaves, flower, and finally 

 fruit in a single season, flowering late a- Mich 

 types do, and beginning to j^row upward late. 



The marsh plants and Hygrophiles generally 

 are similarly retarded. It is then-fore unlikely 

 that many aquatic plants should be annual. 

 For the possible success of the flowering cycle, 

 and maturing of seed, may not eventually 

 result in the propagation of a new plant from 

 seed next spring. Therefore the provision \ 

 perennial underground organs or roots or 

 rhizomes is a great assistance to aquatic plants. 

 The time taken in developing these must equal 

 the ordinary life of an annual. 



Moreover, the reproduction of aquatic plants 

 is very largely vegetative. Resting buds or 

 hibernacula are formed ill winter by Frogbit. 

 Pondweeds, &c. Naturally the aquatic element 

 is for these purposes a distinct advantage. 

 Normally, of course, growth is going on 



