SCIENTIFIC COMMITTEE, AUGUST 19. 



cxlvii 



such as Cabbage, £c." Some doubts were expressed as to the desirability 

 of using arsenical preparations, in case it might be absorbed by the Onions 

 when eaten in the early stage. 



Physianthus and Moths. — Prof. Henslow exhibited flowering sprays 

 of this plant from Cape Town, in which nearly every flower had retained 

 a grey moth, its proboscis having been firmly caught between the anthers, 

 which are fixed to the stigmatic head in Asclepiads. The moths either 

 died from starvation or were picked off by bats, which are aware of their 

 constant presence in these particular flowers, and hunt for them therein. 



Pelargoniums with secondary tubers. — In allusion to the illustration 

 lately received of Leucojum with secondary tubers below the first, Prof. 

 Henslow showed specimens of small tuberous-rooted Pelargoniums from 

 the slopes of Table Mountain, in which similar secondary tubers were found 

 below those from which the foliage and rlowers proceeded. He suggested 

 that they might be water-reservoirs in this particular case, as the plants 

 were in full flower in the dry season. Such tubers occur in other plants 

 (as species of Erodium) in the South African deserts. 



Phototropism.—Vroi. Henslow described an experiment to illustrate 

 the effect of light in connection with gravity, &c. Mustard-seed was 

 grown on a thin layer of cotton wool, kept moist, on a perforated tin 

 tray, suspended under an inverted flower-pot raised upon a support so 

 that the Mustard was illuminated only from below by means of a sheet of 

 white paper. Gravity (the attractive force of the earth) had no effect 

 upon the germinating radicles. If any protruded through the holes they 

 at once turned back, and with all the rest were entwined in the wet wool. 

 Hydrotropism thus entirely superseded gravity. When the tin was 

 suspended horizontally, the hypocotyls or caulicles with the green 

 cotyledons curved downwards ; those on the circumference, being more 

 strongly illuminated, curving more rapidly than the cluster in the 

 middle. AYhen the tin was suspended vertically, after two or three days 

 all the seedlings curved downwards in the direction of both light and 

 gravity, phototropism overcoming negative geotropism, or apogeotropism. 

 These terms, of course, only describe the movements of the hypocotyls 

 as "towards the light," or "in opposition to gravity," or " away from 

 the earth." They are not " forces." The experiment suggested the 

 possibility of different degrees of illumination being the primary in- 

 fluence in causing the upward growth of the stem and the downward 

 growth of the root. It may be remembered that aerial roots of Ivy 

 protrude on the less illuminated side, irrespective of gravity. So, too, 

 the radicle of Mistleto grows towards the bough on which the seed is 

 fixed ; so that as the direct light from the sky is greater than the 

 reflected light from the soil, the shoot-end of a plant grows upwards 

 and the root-end downwards. Gravity, however, is believed to act upon 

 the root-tip, as Darwin and his son, Dr. Francis Darwin, explain, unless it 

 be overcome by the presence of water, manure, &c. The stem, by growing 

 upwards in opposition to gravity, puts out mechanical tissues to support 

 itself under the influence of gravity, which always tends to pull it 

 down, and has acquired a permanent tendency to rise, as shoots laid 

 horizontally will rear their tips perpendicularly if kept in total darkness. 

 Similarly, it is presumably possible that the root- tip has become sensitive 



