TUBERS OF THLADIANTHA DU HI A. 517 



long and thin root-fibres subterranean tubers of a size varying between that of a 

 Hazel-nut and that of a medium-sized Potato. These tubers are swellings of the 

 thin root-fibres, and the growth in length of the latter is not interrupted by their 

 formation. Consequently we find in the middle of a root-fibre 1-2 metres long, 

 one to three tuberous swellings, the transverse section of each being approximately 

 quadrangular, with rounded corners ; the tubers are thus four-sided prisms, and 

 usually lie in the soil in such a manner that one of the flat sides is uppermost, and 

 the other turned downwards. In Autumn, the plant together with its root-fibres 

 perishes, and only the tuberous swqllings of the latter persist in the soil through the 

 Winter. In April we find on these, numerous thin root-fibres proceeding from all 

 sides, as well as young shoots 1-2 cm. in length : it is with the latter we are here 

 concerned. If the tuber lies horizontally in the earth, all the shoot-buds arise on 

 the upper side : if, on the contrary, the acroscopic end of the tuber (i. e. the end 

 directed towards the apex of the root) is directed upwards they are all situated on this, 

 around the point whence the root-fibre continued its growth. To me this fact leaves 

 no doubt that we are here concerned with an effect of gravitation on the production 

 of the growing points of the shoots, since light was excluded, the object remaining 

 underground. It should also be mentioned, however, that in all cases the shoot- 

 buds arising from the tuber are more crowded towards its acroscopic end. A large 

 number of experiments ^ which I made with the tubers of Thladiantha dubia, yielded 

 distinctly the result that the shoot-buds are formed exclusively on that side of the 

 tuber which hes uppermost (towards the zenith) during their development, and that, 

 moreover, in virtue of an internal disposition the acroscopic end of the tuber is 

 preferred in the formation of buds. Here, therefore, are two different causes 

 working together, and determining the place of origin of the adventitious growing- 

 points of shoots — internal causes bring it about that the end of the tuber situated 

 towards the apex of the root is especially qualified for the formation of buds, while 

 at the same time the influence of gravitation determines that the buds arise on the 

 side of the tuber which is turned away from the centre of the earth. 



Not less instructive are my observations on the genus Opuntia, one of the 

 best known forms of Cactus. In Opuntia ficus Indica and O. crassa, with which 

 I experimented, the sub- aerial vegetative body consists of elongated flattened 

 segments of the shoot-axis, on which the positions of the suppressed leaves 

 are indicated only by tufts of prickles. Under normal conditions of vegetation, 

 new flattened segments arise chiefly from the apex of the segments which then 

 happen to be uppermost, and especially from their margins or angles, the angles 

 directed upwards being always preferred in the process. It is easy to see that 

 an impulse exists in the plant which leads to the appearance of new shoots 

 above, and at the same time preferably at the narrow angles of the flat segments : 

 it is only extremely rarely that a new shoot makes its appearance on the flat 

 broad side of an old segment. The tendency for new shoots to appear chiefly at 

 the apical portions of the older ones, is in fact very common in plants generally, but 



' More details as to the behaviour here described of the root-tubers of Tliladiantha dubia and 

 of the shoot segments of Opuntia are found in my treatise ' I'hcr Stoff tttid Form der Pßanzen- 

 organe' Arb. des bot. Inst, in Wzbg. B. II, 1882, p. 698. 



