UPON THE COLOUR OF THE RIVER SPONGE. 261 



upon a stone ; it was somewhat lobed, but of a pale buff, or 

 yellowish-brown, and not unlike, in colour, the common offi- 

 cinal sponge when dry. It was entirely destitute of any 

 green tint, and was taken by me from the bottom of the 

 rivulet under the low and narrow arch of a small bridge, 

 where, in truth, the rays of the sun could, even on a 

 summer's day seldom or never penetrate. Having carried 

 home this mass in a vessel of water, I immediately placed 

 it in a basin of water, adding fresh daily, as I had done 

 in the former experiment, and put the basin in a window 

 facing the west, so that the sun might shine through the 

 glass fully upon it : after a short period, I saw the extre- 

 mities or tips of the lobes of the sponge begin to be slightly 

 coloured, and in a few days I had the satisfaction of ob- 

 serving the lobes to have received a distinct green, which 

 gradually commenced spreading over the lower portions of 

 the mass that were further apart from the direct and nearer 

 influence of the light. It was consequently apparent, that 

 the want of green colour in this mass proceeded from the 

 absence of light, at least, of a sufficient quantity of light, 

 in the original spot, underneath the low and dark arch, where 

 the specimen was found growing. (3) Another experiment will 

 also establish the fact, that light is requisite to increase the 

 green colour already received by this substance ; namely, I 

 procured a piece of greenish or very light-green sponge (4) 

 attached to a small stone, which I immersed in a dish of 

 water, and similarly placed it, as in the former experiments, 

 before the brightest light in the same window : the colour 

 continued to increase in intensity, until, at the end of three 

 weeks, it had become of a beautiful and dark emerald green. 

 Thus, then, from these observations, I think no doubt can 

 be entertained, that light is absolutely necessary, not only to 

 produce the green colouring matter in this kind of sponge, 

 but likewise to increase the intensity of the green itself, in 

 various degrees, in proportion as it i^more or less powerfully [5) 

 acted upon by the addition of the sun's rays. 



Therefore the opinion advanced sometime since by Lamou- 

 roux, can no longer be considered tenable ; " leur couleur^'' 

 says that eminent zoophytist, " est un vert plus ou moins 

 fonce, qui semble varier suivant la nature du corps auquel 

 elles adherent.''' ('Hist, des Polyp. Corall. Flex.,' p. 5). 



Now, the influence of light upon the colouring matter of 

 this Spongilla, proved by the experiments shortly here de- 

 tailed, will strike every botanist as being precisely analogous 

 to the same cause, which effects the like phenomena, under 

 similar circumstances, on the stems and leaves of plants ; 

 and therefore, may have much weight in determining the true 



