174. 



PRINCIPLES OF PLANT-TERATOLOGY. 



Forking of the leaf is a normal feature in some ferns, 

 e. g. Schdz&a, Dipteris, Platy cerium, and some species of 

 Ophioglossum, and was possibly much more prevalent 

 in the past ; hence the abnormal dichotomy of ferns, 

 which in a normal way are monopodially branched, 

 is probably a reversionary phenomenon, as in the 

 adder's tongue {Ophioglossum vulgare) whose ab- 

 normally-forked frond perhaps harks back towards 

 the primitive type, of which the normally-forked leaf 

 of 0. palmatum represents a more accurate reflexion. 

 Forking in fern-leaves is probably rendered more 

 frequent owing to their prolonged apical growth. As 

 will be seen below, it is not of the same morphological 

 nature as the forking of leaves of Phanerogams. 



On the other hand the forking of cotyledons should 

 be regarded as not a reversionary, but a progressive, 

 phenomenon, as also that of many cases of forking 

 of opposite-decussate foliage-leaves. 



But a full discussion of these phenomena of fork- 

 ing will be reserved until the subjects of Phyllotaxis 

 and the abnormalities of the flower are considered. 



As regards the change from simpler to more com- 

 pound leaves, this is almost certainly a reversion 

 towards ancestral conditions. The normal clover-leaf 

 is probably merely a reduced form of the pinnate leaf 

 found in so many other Leguminosse, and the trans- 

 formations it undergoes, as described above, are 

 changes in the direction of more advanced pinnation. 

 The abnormal leaves are often formed as a result of 

 mowing the first-formed leaves off, and the rapid and 

 vigorous formation of new leaves thus caused seems 

 to induce the change ; also these leaves are more often 

 formed during the more vigorous growth of early 

 summer than during the later months of that season. 



The appearance of bipinnate leaves in place of 

 phyllodes in certain Australian acacias is a very clear 

 instance of reversion to the primitive condition. The 

 ontogeny of the seedling of these acacias exhibits 

 these bipinnate leaves at a certain stage. And all the 



