190 



PRINCIPLES OF PLANT-TERATOLOGY. 



leaves may become trifoliolate, or the still greater 

 change may occur, and a large simple leaf be produced, 

 showing no sign of dissection. Tn the ash an entire 

 tree may exhibit none other but these simple leaves; 

 it is known as the variety monophylla or intcgrifolia. 

 The simple leaf so developed does not represent merely 

 the terminal leaflet of the normal pinnate leaf, but 

 stands rather for the whole leaf in which a congenital 

 fusion of all the leaflets must be assumed to have 

 occurred. The simple-leaved variety of Sarnbucas nigra 

 has its counterpart as a normal feature in certain forms 

 of 8. javanica where the leaves are perfectly simple. 



There must next be cited instances of simplification 

 of a leaf which is not compound, but merely deeply 

 segmented. The hawthorn {Crataegus Oxyacaittlio) ex- 

 hibits great variety in its foliage; the variety lacimiata 

 has very deeply-incised leaves ; other leaves are quite 

 simple and hardly dissected at all ; and all transitions 

 occur between these two extremes. In the foliage 

 of 0. sinaica a most interesting and instructive series 

 of forms was observed. The normal or typical leaf 

 consists of a deeply trifid blade and two very small 

 pseudo-stipules* situated at the base of the petiole and 

 separated from the blade by quite a quarter of an inch. 

 In other leaves the pseudo-stipules were considerably 

 enlarged, the leaf -blade was reduced, correctively 

 therewith, in size, and pseudo-stipules and leaf -blade 

 were much more closely approximated to each other. 

 In other leaves the pseudo-stipules were in the form 

 of leaf -lobes resembling in every respect the other lobes 

 of the leaf and perfectly continuous with them. In 

 still other much smaller leaves the pseudo-stipules had 

 become completely merged into the leaf -blade, which 

 exhibited no trace of lobing, but was simply dentate, 

 and extended to the base of the rachis as far as the 



* This term is used because Domin has now clearly shown that true 

 stipules are in all cases morphologically derived from the leaf-sheath and 

 not from the leaf-blade. Good instances of pseudo-stipules are seen in 

 Crsetagus (as shown above), Chrysanthemum, and Cineraria ; these structures 

 are always outlying segments of the leaf -blade. 



