338 METAMORPHOSIS 



the leaf-primordia are not so obvious as this, and they are certainly not 

 * mechanical '. As to the distribution of pressure on the cone above the youngest 

 leaf-rudiments we know nothing. Recently Leisering (1902), a pupil of 

 ScHWENDENER, has directly proved that in these cases there is no external 

 pressure exerted on the growing point. When Leisering has recourse to 

 internal tensions induced by the base of the projecting leaf, the theory is placed 

 in a position where science surrenders the field to imagination. Any detailed 

 criticism of the mechanical theory need not be entered into here. (Compare 

 Schwendener, 1883, &c. ; Vochting, 1894 and 1902 ; Winkler, 1901 and 

 1903.) We need only add that in our opinion pressure has practically no 

 significance as regards the arrangement of organs. [Compare Falkenberg (1901, 

 41), and Berthold (1904, 133).] 



In those cases also, such as axillary buds, where there is nothing against 

 the acceptance of the existence of pressure on the growing point, it is still an 

 arbitrary proceeding to regard this pressure as the only adequate factor and to 

 ignore all the other relations of the organs, which must exist on the analogy of 

 the numerous examples of correlation known. In addition it may be mentioned 

 that in not a few cases the same arrangement of lateral organs may be observed 

 when these are placed at a distance for those next older, where contact is impos- 

 sible (compare Fig. 65, p. 274), and where Leisering's internal tensions cannot 

 exist. If, however, in such well-established cases contact plays no part it is 

 not unnatural to suspect that it plays no part in others also. 



Further, the arrangement of organs is not always determined by adjacent 

 organs ; the arrangement of lateral roots is known to be determined by internal 

 anatomical structure, and it may be imagined that at the growing point of the 

 stem, and in flower-buds especially, similar conditions occur. In Campanula 

 media the orientation of the carpels follows that of the calyx, and, according to 

 the number of the whorls which are interpolated between the calyx and the 

 carpels, the latter alternate with, or are opposite to, the stamens (Eichler, 

 Bliitendiagramme, I, 295. How the carpels come to know the arrangement of 

 the sepals is the puzzle. 



It may be doubted whether the mechanical theory is in the first place 

 purely mechanical, and in Leisering's most recent publication the possibility of 

 a stimulus effect of the supposed pressure is somewhat more clearly suggested. 

 That such activities may exist no one will deny, but proof of their existence at 

 the growing point is not as yet forthcoming. Although we may be unwilling 

 to accept the mechanical theory of leaf arrangement we must at the same time 

 admit that we have nothing better to put in its place, nor do we gain much 

 by referring the arrangement of organs to correlations of growth in general terms. 

 At all events it may be noted that there are certain regular arrangements of 

 organs in the plant which must also be referred to correlation, such as the dis- 

 tribution of the xylem groups in the transverse section of the root and of scleren- 

 chyma and assimilatory tissue in the cortex of the stem. No structures are 

 fixed hereditarily once and for all, they are in the highest degree variable. 

 Doubtless every cell in the periphery of the central cylinder of the root is 

 capable of forming a vessel, just as every cell below the growing point is capable of 

 forming a leaf ; nevertheless, only certain cells at regular distances from each 

 other become vessels and certain regions on the growing point become leaves. 



The relations of the parts to each other and to the whole determine the 

 path of development of each individual plant organ, it is not predetermined 

 — that is the sum and substance of our interpretation of correlations. 



It was remarked at the beginning of the present lecture that the phenomena 

 of correlation could be regarded as internal or as external factors in plant 

 formation. In the case of a complicated plant internal influences can scarcely act 



