CORRELATIONS 



337 



1 



ones is due to the fact that too much water is withdrawn from them by the chief 

 buds (compare Wiesner, 1889) ; but it is more probable that we have to con- 

 sider in this relation not merely water but to look generally for the factors in 

 such correlations in their relation to plastic materials. The case becomes much 

 more difficult when we come to deal with fundamental qualitative changes ; as 

 to the factors concerned in such changes we shall have something to say in the 

 next lecture. 



From Goebel's researches (1903) we may conclude that he was unable to 

 refer the determination of the position of innovations to a polarity as yet by no 

 means understood. According to Goebel, the direction of the circulation of plasta 

 is of much greater consequence. Roots and stems show innovations at their 

 apices and at other places towards which the stream of plasta generally trends ; 

 the leaf, however, shows regenerations at the base also, because the stream of 

 assimilation products is directed backwards. With the object of supporting 

 his view Goebel carried out a number of interesting experiments, but these 

 were by no means exhaustive ; much has yet to be done on the subject. 

 Still, we are acquainted with many facts which are not in accord with Goebel's 

 conception. Basal innovations, for example, appear on the leaf and flower axis 

 of Achimenes if these be treated as cuttings, and yet the course of migration of 

 plasta in these organs is quite different (Hansen, 1881). 



Although we may be inclined, taking it altogether, to consider correlations 

 as due to plastic influences of some sort yet we must not say that mechanical 

 influences are entirely excluded ; at least in the case of organs which touch 

 each other such effects are quite possible. A distinction must be made between 

 a purely mechanical pressure and a stimulus effect such as we referred to in a 

 previous lecture on pressure, contact, &c. The present opportunity may be 

 taken of saying a few words on mutual mechanical influences of organs. 



Very frequently we meet with pressure at the growing point in the course 

 of normal development, when the young members exhibit more vigorous growth 

 than the bud-scales, or, to speak more generally, than the external foliar organs 

 which enclose these young members. Many special bud arrangements are due 

 to internal factors exclusively, others are due to space relationships. The 

 crumpled petals of the poppy, for example, broaden out when the calyx is 

 removed, but remain crumpled if again enclosed in a shell of plaster of Paris or 

 other artificial calyx (Arnoldi, 1900). Such unimportant and for the most 

 part transitory effects are limited to the mutual pressure of organs at the growing 

 point ; only rarely are they seen in full-grown organs, as in the inwardly directed 

 pressure of outer leaves of Agave on inner ones. As Hofmeister has shown 

 (1868, p. 638) there is no case known where the formation of an organ is essen- 

 tially conditioned by mechanical influences. What is true of formation is also 

 true of position. A mechanical theory of leaf -arrangement has been established 

 by Schwendener (1878), and elaborated still farther by his pupils ; he believed 

 that mechanical relationships, especially the distribution of pressure at the 

 growing point, determine the point of origin of new organs, and further that 

 mutual pressure of organs may cause alteration in the primary arrangement. 

 Alterations in position, such as those Schwendener seeks to explain, are, 

 however, nowhere observed, and they would also be absolutely unintelligible 

 (Schwendener, 1883, &c. ; Schumann, 1899 ; Jost, 1899, &c.). 



Evidence in support of the assertion that the primordia of organs are 

 conditioned by pressure is as yet entirely wanting. In individual cases, e.g. 

 the formation of organs in axillary buds, the mechanical theory, although not 

 proved, appears to us to be simple and clear. Since a pressure may frequently 

 be produced on opposite sides of the growing point of the axillary bud by the 

 subtending leaf and the axis, and if new leaves arise at the point of least pressure, 

 they must appear laterally ; but the relationships of any growing point bearing 



