CLIMBING STEMS. 669 



ditches amid reeds and rushes, where a great agitation of the water has never to 

 be provided against. 



In similar habitats other species of the last group of plants with floating stems 

 are also found, viz. those known as " swimming " plants (plantce natantes). They 

 are distinguished from floating plants especially by the fact that their green foliage 

 and in part their stems also lie on the surface of the water, and are in contact on 

 the upper side with the air, or even rise above the water, when they are completely 

 surrounded by air. The stem rests and moves on the surface of the water, and is 

 never held fast in the muddy bottom — even when roots are present. Amongst the 

 well-known forms belonging to this group are several Duckweeds {e.g. Lemnapolyr- 

 rhiza, gibba, minor) with stems curiously flattened and leaf-like. Besides these, 

 there are Salvinia and Azolla, belonging to the vascular cryptogams, and finally 

 several species of Pistia, Pontederia, and Desmanthus, belonging to tropical 

 waters. It has already been mentioned (p. 638) that the floating capacity of 

 Pontederia crassipes is increased by the possession of a vesicular, air-containing 

 tissue in its swollen leaf-stalks. Moreover, in Desmanthus natans an actual swim- 

 ming apparatus is developed, not in the leaf-stalks, but in the stem itself. It takes 

 the form of a large-celled, spongy, air-containing mantle, arising here below the 

 epidermis of the internodes which renders sinking impossible. The mimosa-like 

 foliage-leaves rise up from the nodes of these floating stems like masts with flags. 

 When the leaves turn yellow, the stems rid themselves of their swimming organs 

 which are no longer needed, and indeed it appears to be an advantage to the leafless 

 stems to be able to sink down and to obtain a period of rest at the bottom. 



Several species of the last group of plants with floating stems strongly remind 

 us of plants with procumbent stems. At the stem-nodes they develop roots which 

 sink into the depths, and green leaves which rise up to the sunlight, and the only 

 difference consists in the fact that in the one case the water, and in the other the 

 soil, forms the bed, and even this distinction is sometimes obliterated. When the 

 level of the water sinks, the floating plants sink with it, till finally they lie on the 

 mud, and then, as a matter of fact, they are scarcely distinguishable in habit from 

 plants with procumbent stems which grow on the soil of the moor. 



CLIMBING STEMS. 



Often it happens that the name of a plant aflfects our imagination by its pleasing 

 or harmonious sound. One associates with the name not merely the idea of the 

 form of a certain plant, but more than this, its whole surroundings, framed in which 

 it grows and flourishes. One conjures up a picture of a flowery meadow or scented 

 wood with which the plant with pleasing name can only harmonize. It may be 

 some far-back reminiscence is bound up with the pretty name, or we have read a 

 vivid description in a book long ago. Thus idealized, one shrinks from approaching 

 it with critical eye, from examining it with knife and microscope, and from classify- 

 ing and describing it in the dry language of the specialist. 



