CHAP. IX. TKANSMITTED EFFECTS OF LIGHT. 471 



a lateral light, but not nearly so much as the many other seed- 

 lings in the same pots. This latter case shows that cutting off 

 the tips does not by itself injure the plants so seriously as to 

 prevent heliotropism ; but we thought at the time, that such 

 injury might follow when a greater length was cut off, as in the 

 first set of experiments. Therefore, no more trials of this kind 

 were made, which we now regret ; as we afterwards found that 

 when the tips of three cotyledons were cut off for a length of 

 2 inch, and of four others for lengths of -14, -12, !, and -07 

 inch, and they were extended horizontally, the amputation did 

 not interfere in the least with their bending vertically upwards, 

 through the action of apogeotropism, like unmutilated speci- 

 mens. It is therefore extremely improbable that the amputation 

 of the tips for lengths of from '1 to '14 inch, could from the 

 injury thus caused have prevented the lower part from bending 

 towards the light. 



We next tried the effects of covering the upper part of the 

 cotyledons of Phalaris with little caps which were impermeable 

 to light ; the whole lower part being left fully exposed before a 

 south-west window or a bright paraffin lamp. Some of the caps 

 were made of extremely thin tin-foil blackened within; these 

 had the disadvantage of occasionally, though rarely, being too 

 heavy, especially when twice folded. The basal edges could be 

 pressed into close contact with the cotyledons ; though this 

 again required care to prevent injuring them. Nevertheless, 

 any injury thus caused could be detected by removing the caps, 

 and trying whether the cotyledons were then sensitive to light. 

 Other caps were made of tubes of the thinnest glass, which 

 when painted black served well, with the one great disadvantage 

 that the lower ends could not be closed. But tubes were used 

 which fitted the cotyledons almost closely, and black paper was 

 placed, on the soil round each, to check the upward reflection of 

 light from the soil. Such tubes were in one respect far better 

 than caps of tin-foil, as it was possible to cover at the same 

 time some cotyledons with transparent and others with opaque 

 tubes ; and thus our experiments could be controlled. It should 

 be kept in mind that young cotyledons were selected for trial, 

 and that these when not interfered with become bowed down 

 to the ground towards the light. 



We will begin with the glass-tubes. The summits of nine 

 cotyledons, differing somewhat in height, were enclosed for 

 rather less than half their lengths in uncoloured or transparent 

 21 



