INFLUENCE OF EXTERNAL STIMULI. GRAVITY 221 



influence. Leitgeb also states that the influence of gravity is felt in 

 the earliest stages of development in the prothalli of the homosporous 

 ferns. In Ceratopteris, for example, the cell-surface of the young 

 prothallus is vertical and the two-sided apical cell forms segments 

 alternately towards the zenith and towards the apex ; dorsiventrality 

 does not yet exist. It appears to me doubtful however whether other 

 factors do not operate here, such, for example, as hydrotropism. 



More striking is the influence of gravity on the disposition of the 

 shoots of higher plants l . Flat-stemmed species of Opuntia, for example, 

 O. Ficus-indica, produce their later shoots chiefly on the apical parts 

 of the older segments and as a rule out of the edges. That this con- 

 tinuous production of the shoots in the upper parts is an effect of 

 the influence of gravity is shown by the fact that if the segments of the 

 stem are inclined obliquely new shoots issue from the fiat side which 

 is directed upwards. This only takes place however after prolonged 

 influence. I have found also the lateral shoots issuing upon the upper 

 side only of shoots of Echinocereus cinerascens lying on the ground. 



The tubers of Thladiantha, one of the Cucurbitaceae, behave in a 

 similar manner. They arise as swellings of the thin root-threads, and 

 in the year after their origin they form adventitious shoots which appear 

 above ground. These shoots arise, as Sachs observed, exclusively upon 

 that side of the tuber which is directed towards the zenith at the time 

 when they are formed ; moreover the ' acroscopic ' end of the tuber, the 

 one directed to the point of the root, has a preference in respect of these 

 in conformity with its ' inner disposition ' a state of matters opposite to 

 that which commonly holds when formation of shoots takes place on portions 

 of root 2 . Two kinds of causes then are working here together and 

 determine the place of origin of the vegetative points of the adventitious 

 shoot : ' internal causes,' the result of the direction in which the plastic 

 material moves, bring it about that the ' acroscopic ' end of the tuber 

 is usually selected for the bud-formation, just in the same way as in 

 the shoot-tubers of the potato the apical end is preferred ; and at the 

 same time the influence of gravity causes the shoots to arise upon that 

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



The phenomena we have described in Thladiantha lead us naturally 

 to the consideration of the influence of gravity upon the process of 

 regeneration. 



The most simple case is that in which. regeneration takes place upon 

 a severed portion of a shoot provided neither with primordia of shoots 

 nor with primordia of roots, such, for example, as a long internode 



1 Sachs, Stoff und Form der Pflanzenorgane, Gesammelte Abhandlungen, ii. p. 1 1 59. See also 

 Sachs, Lectures on the Physiology of Plants. 



2 See p. 44. 



