ADAPTATION TO WATER 153 



experiments with. Ranunculus sceleratus, in which the latter was 

 grown under varying conditions of water content. The leaves 

 of the mud and floating forms were found to be larger than those 

 of the drier soils, but they had changed little or not at all in thick- 

 ness. The lobing of the leaves was also found to be reduced, or 

 the lobes often came to overlap. Water plants rarely show any 

 modifications of epidermis or stomata, which could serve to hinder 

 transpiration. Stomata are usually more numerous upon the 

 upper surface, where they are completely exposed. In the same 

 species their number is greater in the forms grown in wet places. 

 The air spaces are extremely large, and, in connection with the 

 abundant stomata, permit of very rapid transpiration. The 

 increase in the amount of air space is accompanied by a reduction 

 of palisade tissue, and a decided increase in the sponge tissue. 

 The result is to expose the chloroplasts more completely to the 

 sunlight, and to augment the consequent loss of water. The 

 complete absence of storage tissues is a further indication that 

 the leaves of water plants are adapted to promote water loss. 



Experiment 47. Experimental adaptation to water. The most satis- 

 factory plants for experiments in adaptation are found in plastic species, 

 such as Ranunculus sceleratus, and the so-called heterophyllous ones, 

 e.g.. Ranunculus delphinifolius, Roripa americana, etc. These are 

 amphibious, i.e., capable of growing in water or on land, and are con- 

 sequently able to undergo adaptation in both directions. Seeds of 

 these plants may be collected in the field, or the plants may be trans- 

 ferred to the greenhouse and allowed to mature there. Ranunculus 

 sceleratus is especially suited to work of this sort, since it grows and 

 reproduces with the greatest readiness in the plant house. 



The seeds are germinated under the usual conditions beneath glass 

 or sphagnum. After the plantlets have developed four or five leaves, 

 thirty or forty are transplanted into 2-inch pots. The transfer to 4-inck 

 pots is made about the time the leaves reach ten or twelve in number. 

 The plants are still kept under uniform conditions for a few days until 

 they arc well established. They are then subjected to different con- 

 ditions, ranging from soil as dry as possible for growth to submergence 

 beneath the water. Four pots are watered in such amount thai (he 

 plants are just able to make a slight growth. The proper amount can 

 only be found by trial: it is usually from 2.5-50 cc. per day. A second 

 series is watered with twice this amount, approximately 100 cc, and 

 a third one with 200 cc. for each plant. The fourth series is grown in 

 mud covered with a thin layer of water, and the fifth in water, the level 



