RADIAL SHOOTS 



73 



ment of the assimilating surface causes the formation of a cup-like expansion which 

 is a dorsiventral structure, whilst the cylindrical stalk of the cup is radial. This 

 is the case in a special degree in Cladonia verticillata (Fig. 32), in which the 

 stalked cups spring one from the other and give rise to a set of tiers ; and, the 

 edges of the cups being slit in a leaf-like manner, the whole habit of the plant 

 is that of one with whorled leaves somewhat like a Chara. This configuration 

 shows us again how from the most different starting-points similar forms may 

 be reached. 



In plants without chlorophyll there is nothing comparable with all this as the 

 relationship of surface-growth to light is entirely wanting. 



Proceeding now from these general points regarding positions and 

 configurations, the relationships of symmetry of the individual organs will 

 be considered in the following pages under the following headings : 



I. Vegetative organs. 



(a) Shoot. 



(1) Radial and bilateral shoots. 



(2) Dorsiventral shoots. 



(b) Leaf. 



II. Flower and Inflorescence. 



II. 

 POSITION OF ORGANS ON RADIAL AXES. 



The arrangement of lateral organs is in but few cases irregular ; 

 it usually conforms with definite rules. Thus we see on the roots the 

 lateral rootlets arranged in longitudinal rows in correspondence with the 

 anatomical structure. The arrangement of the leaves on the shoot-axis 

 has attracted special attention. This is not the place in which to set 

 forth the facts which have been obtained in the investigations into the 

 position of leaves, and the explanation which the ' spiral hypothesis ' has 

 given to them ; enough has been already said upon the subject and there 

 is no necessity for another account here. The spiral hypothesis is however 

 a complex starting-point, and a comprehensive theory of leaf-positions 

 which shall bring all facts into harmony has not yet been framed. As 

 the mechanical hypothesis of Schwendener limits itself to definite cases, 

 and as further research must hinge upon it, an account of it appears to 

 me to be desirable. Dr. Weisse has at my request been so good as to 

 prepare such a statement. I myself hold another view of the importance 



