ii4 RELATIONSHIPS OF SYMMETRY 



3. I do not mean to say that anisophylly is always directly caused 

 by external factors. It is true that in some cases light and the position 

 to the horizontal produce it, as I shall explain more in detail presently, 

 but in many cases there is no direct relationship to external factors. 

 Elatostemma and Goldfussia glomerata show anisophylly of shoots in 

 their etiolated condition and in every position, although in shoots of 

 G. glomerata growing erect it is less in amount ; and in the anisophyllous 

 shoots of Aesculus and others the anisophylly is already induced in the 

 bud and is not brought about in the course of its unfolding. 



The formation of organs upon lateral shoots can also be influenced, we 

 must note, by the relationships of these to their mother-shoot, relation- 

 ships which still require explanation, inasmuch as the establishing of the 

 relationships in space naturally does not give us any explanation of how 

 they have come about. I have already referred, when speaking of the 

 Thallophytes on page 89, to facts bearing on this subject, for example, 

 that in Antithamnion Plumula the short shoots upon the side of a long 

 shoot turned towards another long shoot remain smaller than those 

 upon the opposite side. 



IV. 



RELATIONSHIPS OF SYMMETRY OF LEAVES. 



Leaves are in the great majority of cases dorsiventral. But they 

 may also be bilateral and radial for example in Iris and Juncus, as has 

 been already pointed out. A consideration of these cases belongs to 

 the special morphology of Spermaphyta, here I have only to mention 

 that a radial construction of the leaves, or one approaching radial, may 

 be brought about by late phenomena of growth if there is produced 

 a leaf-surface nearly equally developed in all directions and placed at 

 right angles to the leaf-stalk. This may happen in two ways either 

 through change of position of the parts of a compound leaf, as, for 

 example, in Marsilia, where the four pinnules are of nearly equal size 

 and radiate from one point ; or by the formation of a peltate leaf in 

 which the leaf-stalk is joined to the under side of the leaf-lamina instead 

 of being directly continued into it, as, for example, in Nelumbium and 

 some other dicotyledonous plants. In the latter case the leaves are solitary 

 and the lamina is at right angles to the orthotropous stalk. The relation- 

 ships to the shoot-axis disappear in a leaf of this kind ; it is to a certain 

 extent an independent orthotropous and radial structure. Thus we 

 find in Cotyledon Umbilicus \ the long-stalked leaves of the rosette are 



1 See Herbert Spencer, Principles of Biology, ii. 



