82 



RELATIONSHIPS OF SYMMETRY 



accurately the degree of pressure to which the bud is subjected on the anterior and 

 on the posterior side, we must seek for aid in this matter from indirect evidence, 

 and especially from that afforded by extreme cases in which the different relationships 

 of pressure can with some certainty be observed. If the lateral shoot in its growth 

 forms almost a right angle with the chief axis, there occurs at a very early period 

 cessation of contact between the mother-axis and the apex of the axillary bud, whilst 

 the contact with the axillant leaf persists for a longer time. In such a case the third 

 leaf will of necessity appear upon the posterior side, and it is found in this position 

 without exception in all plants which possess these conditions. Schwendener has 

 described a large number of cases, especially in the Coniferae and Crassulaceae. 

 In other plants whose axillary shoots are less vertical, this disposition is less constant, 

 as is to be expected. If the axillary bud shoots out from the mother-axis at 



a relatively acute angle, the third 

 leaf is always on the side next the 

 axillant leaf, and this position is 

 characteristic of the majority of 

 dicotyledonous plants. The 

 lateral deviation of the third leaf 

 is brought about by the asym- 

 metric relationships of the leaf- 

 axil, and three of these are of 

 chief importance : 



/ i. A lateral displacement of 



the median of the axillant leaf with 

 respect to that of the mother- 

 stem and the axillary bud. 



2. An oblique insertion of 

 the axillant leaf. 



3. The spiral position of 

 the bases of the leaves of the 

 mother-axis above the axillant 



FIG. 39. Solidago canaclensis. Transverse section of an axillant 

 leaf and its axillary bud. MagniGed 45. After Schwendener. 



leaf through which unequal pressure is exercised upon the axillary bud. 



The first two kinds of asymmetry are common, the third has only been observed 

 so far in the inflorescences of Coniferae. Fig. 39, which is a cross-section through the 

 axillary bud of Solidago canadensis along with its axillant leaf, will enable us to follow 

 the effect of such asymmetric relationships. The median of the axillant leaf lies 

 evidently to the left of that of the stem and bud, which is indicated in the figure by 

 the straight line. Upon this side the axillant leaf will evidently exercise a greater 

 pressure upon the bud than it does upon the right side, and therefore the third leaf, 

 indicated by 2 in the figure, must arise upon the right side. The following leaf, which 

 is towards the posterior side, appears to be displaced to the same side of the median, 

 and the position of the third leaf determines the direction of the spiral : it is in the 

 case before us dextrorse. I must refer for further details to the original works 1 . 



1 Schwendener, Mechanische Theorie der Blattstellungen, p. 98 ; A. Weisse, Beitrage zur mecha- 



