SUBMERGED AQUATIC PLANTS 23 



The structure of an aquatic plant differs from that of a land 

 plant mainly in four respects : 



1. The root-system is either absent or very much reduced. 

 The main function of the root is to fix the plant in the bed of the 

 stream, or on some stone or wood, etc. ; it is not engaged to any 

 appreciable extent in the absorption of food material and in its 

 conduction, as in the land plant. 



2. The length of the shoot is very much longer in a water 

 plant, for the light is less intense, and strong light retards growth 

 in length. A transverse section of the stem of a submerged plant 

 shows a comparatively undeveloped vascular system. There is 

 often very little of the woody tissue which helps to support a land 

 plant (Fig. 8). 



3. The cuticle of the leaves is usually thin, or even absent ; 

 the whole surface is engaged in absorption, and there are no 

 stomata in submerged plants, although they are naturally present 

 in large numbers on the upper surface of the leaves of floating 

 aquatic plants. 



4. Water plants have air spaces, which make them light. 

 Submerged aquatic plants may be arranged in the following 



groups : 



(a) Plants fixed to stones in running water. These include some 

 Mosses and a few tropical plants with very much reduced flowers. 



(b) Rosette plants, rooted in the ground, generally with cylin- 

 drical leaves. These belong to the more or less shallow and still 

 waters of pools and lakes. The Cryptogamic plants, Pillwort 

 and Quillwort ; and the flowering plants, Awlwort and the 

 Water Lobelia, are the best known instances. 



(c) Plants rooted in the soil, or free swimming, with long 

 flowing shoots. The Slender Naiad, common in N. America, but 

 found only in a few places in Britain ; the Hornwort, a free swim- 

 ming form, belonging to pools, and adapted for pollination in 

 water, and the Whorled Milfoil may be mentioned. One or two 

 other plants, as the Water Soldier (Stratiotes) , are almost entirely 

 submerged, and in general habit may be grouped with these. 



The Pillwort (Pilularia globulifera), is not a flowering plant, 

 but bears spores which, unlike those of the fern, are of two sizes, 

 large and small. These are situated in bags or sporangia, which 



