SPONTANEOUS MOVEMENTS IN PLANTS. 285 



Johnson's " How Crops Grow." ' " The cereals are able to dispose of 

 eilica by giving it a place in the cuticular cells; the leguminous crops, 

 on the other hand, cannot remove it from their juices ; the latter remain 

 saturated, and thus further diffusion of silica from without becomes 

 impossible except as room is made by a new growth. It is in this way 

 that we have a rational and adequate explanation of the selective 

 power of the plant." The " rational and adequate explanation " seems 

 to me, on the contrary, to be merely a restatement of this selective 

 power of the tissues in other terms. Because the tissues want the sil- 

 ica, is no explanation of how they get it. 



The curious and interesting movements of climbing plants have 

 been investigated by Palm, Mohl, and Asa Gray, and form the subject 

 of one of the most charming of Mr. Darwin's works. It is well known 

 that climbing plants, such as the hop, honeysuckle, or major convolvu- 

 lus, always twine round the stem or other object which supports them 

 in one direction, that is, always either from right to left or from left to 

 right ; but few, probably, have reflected, and fewer still attempted to 

 observe, by what process the end of the growing shoot contrives to 

 change its position from one side to the other of the stem. If the ex- 

 tremity of a living stem, say of convolvulus, growing perfectly free, 

 and in a normal position, is observed, it is seen to hang over from its 

 support in a horizontal direction ; and this horizontal portion is found, 

 if observed at intervals of some hours, to point in different directions. 

 The end of the growing shoot has, in fact, the property of revolving in 

 a large circle, round the support, always, with the same species, in the. 

 same direction, either with the sun or opposed to the sun. The rate 

 of revolution varies with different plants, and with the same plant at 

 different periods of its growth ; it is much quicker in warmer than in 

 cooler weather. With the hop, Darwin found it to vary from two and 

 a half hours to nine hours. The object of the climbing power of plants 

 is no doubt to reach the light, and to expose a large surface of leaves 

 to its action and to that of the free air ; but the mode by which this 

 power of motion is gained is by no means clear. The late eminent 

 physiologist Mohl supposed that it was caused by a dull kind of irrita- 

 bility in the stem, which caused it to bend toward the support when in 

 contact with it. Mr. Darwin has, however, carefully tested this theory 

 experimentally, and always with negative results. He rubbed many 

 shoots, much harder than was necessary to excite movement in any 

 tendril, or in any foot-stalk, of a leaf-climber, but without result. This 

 view seems also entirely negatived by the fact that not only do the 

 stems of climbing plants revolve when they are not in contact with 

 any support, but even more freely, under such circumstances, than 



1 "How Crops Grow:" A Treatise on the Chemical Compositions, Structure, and 

 Life, of the Plant, for Agricultural Students. By S. W. Johnson. Revised and adapted 

 for English Use, by A. H. Church and W. T. T. Dyer. London: MacmMan & Co., 1869 

 pp. 345. 



