26 



PffYSlOLOG Y. 



the microscope the lower end of the axis, which is attached to 



the substratum, there are seen numerous brown-colored threads 



more or less branched. 



50. Absorption by the higher aquatic plants. Examples of 



the water plants which are entirely submerged in water are the 

 water-crowfoots, some of the pond- 

 weeds, elodea or water-weeds, the tape- 

 grass, vallisneria, etc. In these plants 

 all parts of the body being submerged, 

 they absorb water with which they are 

 in contact. In other aquatic plants, like 

 the water-lilies, some of the pond- 

 weeds, the duck-meats, etc., are only 

 partially submerged in the water; the 

 upper surface of the leaf or of the leaf- 

 like expansion being exposed to the air, 

 while the under surface lies in close 

 contact with the water, and the stems 

 and the petioles of the leaves are also 

 immersed in water. In these plants 

 absorption takes place through those 

 parts in contact with the water. 



51. Absorption by the duck-meats. 

 These plants are very curious ex- 

 amples of the higher plants. 



Lemna. One of these is illustrated in fig. 

 37. This is the common duckweed, Lemna 

 trisulca. It is very peculiar in form and in 

 its mode of growth. Each one of the lateral 

 leaf-like expansions extends outwards by the 

 elongation of the basal part, which becomes 

 ^ng ^d slender. Next, two new lateral ex- 



rhizoids below and the tuft of pans ions are formed on these by prolification 

 leaves above, which protect the r r 



archegonia. from near the base, and thus the plant con- 



tinues to extend. The plant occurs in ponds and ditches and is sometimes 

 very common and abundant. It floats on the surface of the water. While 

 the flattened part of the plant resembles a leaf, it is really the stem, no 

 leaves being present. This expanded green body is usually termed a 



Fig- 36. 



