WATER REQUIREMENT AND ADAPTATION IN EQUISETUM 39 



Equisetum . (table 7). Phaseolus makes 1.6 times as much pho- 

 tosynthate as Helianfhus. The low efficiency of Equisetum may 

 be accounted for by the fact that the stem is the photosynthetic 

 organ rather than the leaves. The position of the stem prevents 

 the rays of the sun from falling directly on the surface so that a 

 given amount of light is spread over a greater area. These 

 slanting rays which reach it are of much less intensity than the 

 direct rays and thus the source of energy for the photosynthetic 

 work is much reduced. 



Helianthus has an epidermis covered with hairs which serve 

 as a light screen and cut out 70% of the light which strikes the 

 epidermis. Phaseolus epidermis screens out 45% of the light 

 incident upon it. The choloroplasts are then functioning not in 

 bright light, even though the leaves are in full sunlight, but in 

 30% and 55% relative light intensity. 



It is interesting to note that the photosynthetic efficiency in 

 these three species is in inverse proportion to the amount of 

 transpiration. 



(4) PHYSIOLOGICAL RESPONSES 



(1) Growth and water content 



A series of Equisetum fluviatile was grown with roots sub- 

 merged, roots in mud, and roots in a water content of 35% 

 from April 2, to May 18. The effect of the varying water con- 

 tents on the size of the plants is shown in plate 5. The following 

 is the height of each: 



Submerged. .80.1cm. Mud 80.5 cm. 35% 68.2 cm. 



As shown on the plate almost no branches developed on the 

 plant in 35% water content. With all the plants used in the 

 greenhouse growth was best in mud, although in many cases 

 the difference between the plants growing in mud and those 

 with submerged roots was very slight. 



(2) Growth and light intensity 



Plants of Equisetum fluviatile were grown in a muslin shade 

 tent which cut down the light to 3% and in the normal light of 



