84 RELATIONSHIPS OF SYMMETRY 



the relative size of their primordia as well as upon the mechanical relationships of 

 their bases. Thus, in a decussation of leaf-pairs the axillary shoots should have 

 relatively large primordia of leaves and submit to nearly equal pressure from 

 the mother-axis and from the bract. The spiral positions finally, are, as has been 

 so frequently stated, always accompanied by asymmetry which exists either at the 

 base of the axis under consideration or appears in its further development. The 

 growth in thickness and in breadth of the young leaf-base appears to be in these 

 cases of medium amount. 



The beautiful results which the mechanical hypothesis of the position of leaves 

 has furnished in the domain of the morphology of the flower ' must be passed over 

 here, because they have to do less with principles than with interesting details with 

 which it is impossible to deal shortly. 



III. 



DORSIVENTRAL SHOOTS. 



The difference between the dorsal and ventral sides of dorsiventral 

 shoots may be expressed in (i) the structure on the two sides, or in 

 (2) the position of tJieir members. 



i. DIFFERENT STRUCTURE OF THE UPPER AND UNDER SIDE. 



The different structure of the two sides of a dorsiventral body is well 

 seen in the thallus of a liverwort, or in a shoot with distichous leaves 

 where the upper surfaces of the whole of the leaves are turned upwards 

 and the under surfaces are turned downwards. In less striking degree 

 the dorsiventrality appears in the shoot-axes of leafy shoots ; but it 

 is quite evident and shows itself partly in the configuration, partly in 

 anatomical structure. The axes of the leaf-like shoots of Phyllanthus 

 lathyroides are flattened on the upper side like many leaf-stalks ; their 

 structure is not unlike that of the axes of many dorsiventral inflorescences 2 . 

 The dorsiventral axes of Hypnum splendens are flattened upon the upper 

 side in like manner. With regard to the behaviour of the Lycopodieae 

 see page 102. In the orthotropous shoots of trees the wood is usually 

 radially developed ; plagiotropous lateral branches very often show the 

 phenomena which C. Schimper described 3 as epinasty and hyponasty, 



1 Schwendener, Mechanische Theorie der Blattstellungen, p. 107 ; K. Schumann, Bliitenmorpho- 

 logische Studien, in Pringsh. Jahrb. xx (1889), p. 349 ; Id., Neue Untersuchungen liber den Bliitenan- 

 schluss, Leipzig '890; Id., Morphologische Studien, Heft i, Leipzig, 1892; A. Weisse, Die Zahl 

 der Randbliiten an Kompositenkopfchen in ihrer Beziehung zur Blattstellung und Ernahrung, in 

 Pringsh. Jahrb. xxx (1897), P- 453- 



2 See Goebel, Uber die Verzweigung dorsiventraler Sprosse, in Arb. d. bot. Instituts in Wiirzburg, 

 ii. p. 430. 



3 C. Schimper, Amtlicher Bericht der Naturforsch.-Versammlung in Gottingen, 1854; Hofmeister, 



