226 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



shoot is restricted by the inverted position, the lateral shoots now more 

 removed from its influence develop under the furthering influence of 

 gravity into orthotropous shoots such as we see occasionally upon prostrate 

 individuals and also upon lateral shoots which become erect to replace 

 a severed apical shoot. Just as gravity is a factor which acts along 

 with the early existing polarity in determining the different construction 

 of the buds in the apical and basal regions of the annual shoots, so 

 also does it act in determining differences between orthotropous chief 

 shoots or chief roots and plagiotropous lateral shoots or lateral roots. 

 The behaviour of some succulent plants in which lateral twigs do the 

 work of the arrested leaves, and, in consequence of this, show a different 

 form from the chief shoots, is specially striking and easy to observe. 

 In Opuntia brasiliensis l the chief shoot is radial and cylindric ; upon 

 it in young plants stand thin flat lateral shoots of about the thickness 

 of a stout pasteboard ; on stronger plants the form of the lateral shoot of 

 the first order approaches the cylindric, only shoots of higher order 

 become flat, leaf-like, and of limited duration. It is easy to show 

 experimentally that the difference in the construction of the shoots is 

 connected with their position in relation to gravity 2 . Suppose that 

 the cylindric chief shoot is severed above a strong flat lateral shoot, 

 this then becomes erect and grows now as a cylindric shoot with 

 the characters of the chief shoot. In like manner flat shoots which 

 are placed as cuttings vertically in soil grow usually further as cylin- 

 dric shoots. Euphorbia alcicornis behaves in quite a similar way ; it 

 possesses a many-ribbed chief shoot and flat lateral shoots upon 

 which the number of ribs is reduced to two ; strong lateral shoots 

 behave here like the chief shoots. If one of the flat two-ribbed 

 shoots be taken as a cutting it grows out into an orthotropous many- 

 ribbed shoot. 



The relationship of the above-mentioned phenomena to anisophylly, 

 so far as our knowledge of the dependence of anisophylly upon outer 

 factors admits, will be discussed below when the influence of light is spoken 

 of. Here I will only say that the earlier view of many authors, like Hof- 

 meister, Frank, Wiesner (the latter has indeed changed his opinion), which 

 made anisophylly a consequence of the influence of gravity, is untenable 

 in its generality, as I have already indicated by the facts brought 

 forward in the chapter upon relationships of symmetry ; for it is 

 unquestionably true that, in many cases at least, anisophylly is directly 

 dependent upon the position of the organ where different factors can 

 influence the different sides unequally. 



1 See Figs. 37 and 38 in my ' Pflanzenbiologische Schilderungen,' i. 



3 See Goehel, in Flora, 1895, p. 113, nnd 'Pflanzenbiologische Schilderungen,' i. p. 74. 



