92 RELATIONSHIPS OF SYMMETRY 



Many creeping and climbing shoots exhibit more or less flattening. 

 The causes of this have not been investigated. It is possible, at least in 

 many cases, that light has a share in it. In connexion with the examples 

 brought forward in the Fifth Section it will be shown that where in roots 

 a flattening takes place under the influence of light a dorsiventral structure 

 is a consequence. In many cases a flattening of the side of shoot-axes 

 and of roots lying upon the substratum evidently results from their pressure 

 upon it when they were still in a juvenile plastic condition, whether this 

 be brought about by negative heliotropism or by other causes. I have 

 elsewhere noticed this feature in many aerial roots of orchids 1 , and it 

 also probably exists in the stems of many ferns, for example, in Acro- 

 stichum scandens where the illuminated side is also flattened (Fig. 48, /). 

 In many ferns the flattening has become hereditary, as in the case of the 

 flat crust-like shoot-axis of Polypodium Schomburgkianum (Fig. 48, //) 

 where the shoot-axes protect the roots. 



FIG. 48. Transverse sections of two flat dorsiventral fern-stems creeping on the baik of trees. / Acrostichum 



scandens. // Polypodium Schomburgkianum. 



B. DORSIVENTRAL LATERAL SHOOTS. 



If lateral shoots are produced upon an orthotropous chief shoot they 

 often have a dorsiventral instead of a radial construction. This may 

 occur in different degrees and be brought about in different ways, and we 

 may make the following grouping of the latter : 



1. Change of the position of the leaves. This takes place 



a. By change of the leaf-insertion. 



b. By torsion of the internode or of the leaf- base. 



c. From the beginning at the vegetative point. 



2. Different configuration of the leaves upon the upper and under side 

 of the shoot (anisophylly). This is often associated with the 

 changes included under the first heading. 



3. Formation of the lateral shoots upon the flanks only of the shoot 

 or preferably there. 



1 See my ' Pflanzenbiolo^ische Schilderungen,' ii. p. 251. 



