564 G. T. HARRIS ON 



oi not more than four. The hut circle, for such it practically is, 

 while not in any way prehistoric is of very considerable antiquity, 

 and probably was constructed as a temporary dwelling, or cache, 

 by tin miners in the days of the tin-streaming industry, as old 

 iurrows are plentiful in the neighbourhood. As will be seen 

 -from the diagram, Schistostega occupies a position in the interior 

 on the south side of the bank of earth, where it faces due north. 

 Here it grew luxuriantly, and was fruiting abundantly at the 

 time of my visit. Now it is obvious that the part of the hut wall 

 to the left of the bed of Schistostega, being to westward, would 

 get less light, while that to the right would have considerable 

 illumination in the late afternoon owing to its facing north-west. 

 A careful examination of the area covered by the moss indicated 

 that its ecological optimum was at the position where it faced 

 north, as on either side its growth thinned off, on the one hand 

 into light of a low, and on the other into light of a relatively high 

 intensity. These mediaeval caches are not infrequent on Dart- 

 moor, more especially to the south of Princetown, and they are 

 almost always associated with a luxuriant growth of Schistostega 

 osmundacea. 



From repeated photometric examinations by Wiesner's method 

 ■(13) I have estimated the specific light-ration of Schistostega to 

 be l/50th of the total unobstructed light, but however carefully 

 ■conducted they may be such estimates can only be an approxima- 

 tion to the actual light-ration of the plant. Wiesner's method 

 depends on the darkening of silver bromide paper to a standard 

 tint, and as the silver haloid used in the preparation of the sensi- 

 tive paper is principally sensitive to the blue- violet rays it follows 

 that a measurement of light made in the open with an abundance 

 of the more refrangible rays cannot compare satisfactorily with 

 A measurement made at a point where the light to which the 

 photometric paper is exposed has the blue-violet rays filtered out 

 to some extent by passing through a screen of foliage more or 

 less dense. 



Small as the light-ration of Schistostega may appear, it is com- 

 paratively generous when compared with that of such a plant as 

 Aspidistra elatior, whose ration is only about l/200thof the total 

 •diffused light outside (14), and it should be remembered that the 

 chloroplasts of sciophilous plants are more sensitive to weaker rays 

 of light than are those of heliophytes (15). The striking bluish 



