340 ON THE GREEN COLOUR OF THE HAIR OE SLOTHS. 



on Sloth's hair quite as much to its exceptional structure as to 

 the humidity of their habitat, and look upon this unusual growth 

 of green parasitic plants as due to the combination of both con- 

 ditions*. If the green colour is really a protection to the 

 animal, one cannot help asking whether the structure of the hair 

 is connected with this protection, either by design or by gradual 

 development. 



On examining the hairs in a natural state with the microscope 

 every stage in the growth of the small green algae can be seen ; 

 but many facts may be better observed by heating the hair in 

 dilute caustic potash. This dissolves the horny substance of the 

 hair, but leaves the alga? more or less free. There is then no 

 difficulty in studying every phase of growth, from green specks 

 °f TOW ^ nca ^ n diameter up to cells of 2 1 o diameter; and 

 they are seen dividing and subdividing in much the same manner 

 as the cells of Chlorococcum, so common on damp walls and trees. 

 Kiihn describes those met with on the hair of Bradypus as differ- 

 ing from those on Cholopus, basing his conclusion, to a great 

 extent, on the number and form of the spores. He names them 

 respectively Pleurococcus Bradypi and P. Cholcepi. The plants 

 certainly differ in several particulars ; but one may doubt whether 

 the difference is not due to difference of conditions. One grows 

 on a comparatively flat surface, which allows of lateral extension ; 

 whereas the other grows in grooves, which allow of only free 

 linear development. We cannot say whether the spores of the 

 one form would grow into the other under changed conditions ; 

 but, at the same time, the different conditions may have led to the 

 production of well-marked hereditary peculiarities. However, 

 whether we call them species or varieties, at all events there can 

 be no doubt that they are minute parasitic plants ; but at the 

 same time I thought it desirable to confirm this conclusion by 

 the independent evidence of spectroscopic examination. 



On heating in alcohol the hair of the Cholopus. fresh from 

 America, the colour was not dissolved, but it was readily soluble 

 after the hair had been boiled in water, and yielded a fine green 

 solution. This I studied very carefully, separating the different 

 constituents in the manner described in my paper on comparative 

 vegetable chromatology f , and compared it with the solution 



[* The hair of Bats, it may be observed, seems adapted for a growth of algae, 

 so far as mere structure is concerned. — Ed.] 



t Proceedings of Royal Society, 1873, xxi. p. 442. 



