We will now turn to the trials with caps made of very thin tin-foil.  These were placed at different times on the summits of 24 cotyledons, and they extended down for a length of between .15 and .2 of an inch.  The seedlings were exposed to a lateral light for periods varying between 6 h. 30 m. and 7 h. 45 m., which sufficed to cause all the other seedlings in the same pots to become almost rectangularly bent towards the light.  They varied in height from only .04 to 1.15 inch, but the greater number were about .75 inch.  Of the 24 cotyledons with their summits thus protected, 3 became much bent, but not in the direction of the light, and as they did not straighten themselves through apogeotropism during the following night, either the caps were too heavy or the plants themselves were in a weak condition; and these three cases may be excluded.  There are left for consideration 21 cotyledons; of these 17 remained all the time quite upright; the other 4 became slightly inclined to the light, but not in a degree comparable with that of the many free seedlings in the same pots.  As the glass-tubes, when unpainted, did not prevent the cotyledons from becoming greatly bowed, it cannot be supposed that the caps of very thin tin-foil did so, except through the exclusion of the light.  To prove that the plants had not been injured, the caps were removed from 6 of the upright seedlings, and these were exposed before a paraffin lamp for the same length of time as before, and they now all became greatly curved towards the light.

As caps between .15 and .2 of an inch in depth were thus proved to be highly efficient in preventing the cotyledons from bending towards the light, 8 other cotyledons were protected with caps between only .06 and .12 in depth.  Of these, two remained vertical, one was considerably and five slightly curved towards the light, but far less so than the free seedlings in the same pots. [page 474]

Another trial was made in a different manner, namely, by bandaging with strips of tin-foil, about .2 in breadth, the upper part, but not the actual summit, of eight moderately young seedlings a little over half an inch in height.  The summits and the basal parts were thus left fully exposed to a lateral light during 8 h.; an upper intermediate zone being protected.  With four of these seedlings the summits were exposed for a length of .05 inch, and in two of them this part became curved towards the light, but the whole lower part remained quite upright; whereas the entire length of the other two seedlings became slightly curved towards the light.  The summits of the four other seedlings were exposed for a length of .04 inch, and of these one remained almost upright, whilst the other three became considerably curved towards the light.  The many free seedlings in the same pots were all greatly curved towards the light.