28o PHYSICAL FACTORS [CH. 



type of leaf shall be produced. For example, the submerged 

 band-shaped leaves of Alisma graminifolium, Ehrh. 1 require a 

 moderate illumination, while the air-leaves flourish in bright 

 light. In shallow water, in which the plants would, under 

 ordinary conditions, form air-leaves, the band-shaped leaves 

 continue to be produced, if the surface of the water happens 

 to be covered with a layer of Algae which reduces the light. The 

 influence of sunshine in this case is perhaps only indirect, the 

 activity of assimilation being probably the critical factor. 



The effect of light upon the germination of the winter-buds 

 of Hydrocharis Morsus-ranae, the Frogbit, has been studied 

 experimentally 2 , and it has been shown that it is impossible 

 for these turions to develop into plantlets, unless they are 

 exposed to a minimum degree of illumination, which is far 

 removed from total darkness. The yellow and orange rays prove 

 to be the most active in promoting germination. But, marked 

 as is the effect of light on the vegetative growth of the Frogbit, 

 its influence in connexion with flowering is far more striking. 

 It has been shown 3 that a set of plants exposed daily from the 

 spring onwards to nine hours of direct sunlight, produced more 

 than a thousand flowers between the end of June and the end of 

 August, while a corresponding set of plants, which were insolated 

 daily for three hours only, produced no flowers at all. Indivi- 

 dual plants from this second set, removed and placed in bright 

 sunshine at the end of June, began to flower in four weeks. By 

 artificially cooling the water in which the insolated plants grew, 

 it was shown that these effects were produced by differences of 

 illumination, and not by the heating influence of the sun's rays. 



Darkness seems to inhibit the germination of certain water 

 plants ; this has been shown in the case of the achenes of Ra- 

 nunculus aquatilts and the nutlets of Callitriche. The seeds of 

 Nymphaea lutea^ also, though they are able to germinate in the 

 dark, do so in far greater numbers in diffuse light. In other 

 cases, e.g. Potamogeton natans^ darkness favours germination 4 . 



1 Gluck, H. (1905). 2 Terras, J. A. (1900). 



3 Overton, E. (1899). * Guppy, H. B. (1897). 



