THE LEAF. 



177 



In Selaginella the seedling-shoot bears at first leaves 

 symmetrically arranged around the axis, and all of 

 one kind ; later on this more primitive type of shoot 

 changes into the dorsiventral heterophyllous type. 

 The same kind of thing has occurred in certain 

 abnormal shoots of 8. grandis which arose in the place 

 of rhizophores on either side of the point of forking 

 of the stem, and also in those which arose as pro- 



Fig. 49. — Convolvulus arvensis. — Foliage of mature shoot showing con- 

 formation typical of cotyledons. (After Siracusa.) 



liferations of the cones ; in each case the shoot showed 

 at first radial symmetry of growth and leaves of one 

 kind ; but eventually as they grew in length they 

 adopted the normal dorsiventral conformation. 



An especially interesting case is that observed by 

 Siracusa in the field bindweed (Convolvulus arvensis). 

 All the leaves of a particular shoot of this plant which 

 grew prostrate on the ground had the bilobed form of 

 the cotyledons (fig. 49). Now the foliage-leaves of 

 the tropical Ijpomsea Pes-Caprse have normally this 

 bilobed form, and the plant, moreover, is normally 



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