RADIAL SHOOTS. MECHANICAL HYPOTHESIS OF LEAF-POSITION 83 



The installation of and the direction of the leaf-spiral are also affected by 

 asymmetric relationships on the axis of the seedling l . In dicotyledonous plants the 

 two cotyledons often show slight deviations from regular opposition and the effect of 

 this is visible in the arrangement of the very first foliage-leaves ; but in other cases 

 the first leaves are more or less regularly decussate, and the decussation only 

 gradually suffers disturbance through the slight irregularities which are always to 

 be observed in organized structures, and is followed usually by a number of 

 leaves irregularly placed until finally a definite spiral position is attained. 



The position of the first leaves in adventitious titrigs is also dependent upon 

 mechanical relationships. Some experimental investigations which I have made 

 with cuttings of willow are so interesting in this respect as to be worthy of note 

 here 2 . If all the axillary shoots be removed from a cutting after it has rooted, 

 adventitious shoots soon form, chiefly on the cut surfaces. By varying the form of 

 the wounded surface, one can frequently cause variation in the base of the 

 adventitious shoot and thus experimentally modify its influence upon the position 

 of the first leaves. The first leaf is always laid down at the place of least resistance ; 

 but with regard to the position of the following leaves further experiment is wanted 

 before a definite conclusion can be come to; they often stand irregularly and it is 

 only gradually, as on the axis of a seedling, that they fall into a definite arrangement. 



The frequent appearance, amidst such changing transitions, of organs in 

 positions referable to the chief series, is partly dependent upon the relative size of 

 the organs, partly upon the basis upon which the system is built 3 . It would 

 however carry me too far to go into this question. 



The relationship of form and of size of the base of the young leaf is the most 

 important element in bringing about in one species regular spiral position of the 

 leaves, whilst in another the distichous or the cyclic arrangement prevails 4 . If 

 the primordia of the leaves grow at an early period predominantly in breadth, that 

 is, in a direction transversely tangential to the apex of the stem, so that they embrace 

 more than a half of it before the following leaf shoots out, there arises, if there be 

 symmetric construction of the sides of the leaf, a distichous arrangement. On 

 the other hand, if the growth of the young primordia is predominant in thickness, 

 that is to say, in a direction radial to the apex of the stem, then generally leaf-pairs 

 and whorls arise. The number of members of any one whorl is dependent upon 



nischen Theorie der Blattstellungen an Axillarknospen, in Flora, 1889, p. 114; Id. Uber die Wen- 

 dung der Blattspirale nnd die sie bedingenden Druckveihaltnisse an den Axillarknospen der Coniferen, 

 in Flora, 1891, p. 58. 



1 Schwendener, Wechsel der Blattstellungcn an Keimpflanzen von Pinus, in Sitavngsber. cl. bot. 

 Vereins d. Provinz Brandenburg, xxi (1879), p. 109 ; Bernh. Rosenplenter, Uber das Zustandekommen 

 spiraliger Blattstellungen bei dikotylen Keimpflanzen. Inaug.-Dissertation. Berlin, 1890. 



2 A. Weisse, Neue Beitrage zur mechanischen Blattstellungslehre, in Pringsh. Jahrbiicher, xxvi 

 (1894), p. 238. 



3 Schwendener, Zur Theorie der Blattstellungen, in Sitzungsber. der Berliner Akad. d. Wissensch., 

 1883, p. 750 ; A. Weisse, Neue Beitrage zur mechanischen Blattstellungslehre, in Pringsh. Jahrb. xxvi 

 (i 894), p. 256. 



4 A. Weisse, Neue Beitrage znr mechanischen Blattstellungslehre, in Pringsh. Jahrb. xxvi (1894), 

 p. 236. 



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