1 86 IX THE SHOOT AS A WHOLE 



it does not follow from its acceptance in these cases that a theory of 

 recapitulation can be applied consistently, or in detail, to all phases of 

 development, or that evidence of it is to be found necessarily in the 

 earliest steps of the embryogeny. It remains for the morphologist to draw 

 for himself the reasonable limit of its application. If this be done, and 

 especially if the variability which exists be duly appreciated, then the early 

 stages of the initial embryogeny^ of the sporophyte will take their right 

 place: and recapitulation will be traced as a limited phenomenon^ only, 

 applicable, it is true, to the case of relatively recent adaptations, but not 

 with equal certainty to the far-away facts of the past. For reasons such 

 as are explained in this chapter, it will not be assumed that plants so 

 diverse as are the main groups of Archegoniatae show in their early seg- 

 mentation, or in the initial form of their embryos, any detailed reflection of 

 ancestral characters. The facts observed should be used with the greatest 

 caution, especially where the comparisons are made between representatives 

 of phyla which must have diverged early from some primitive stock, if 

 indeed they be related at all. 



Certain points touched in the above discussion will help towards an 

 understanding of the relations of sporophylls and foliage leaves to the 

 first leaves of the embryonic plant. In Chapter XIII. it was concluded 

 that in certain cases at least foliage leaves are to be held phylogenetically 

 as sterilised sporophylls : and the question remains whether or not all 

 non-propagative leaves, including the cotyledons themselves, originated in 

 this way. There seems to be a high probability that in the Pteridophytes 

 they did. There is no reason to hold that their first leaves differ in any 

 essential point from those which are formed later : frequently they resemble 

 the later leaves closely in outline ; but they are sometimes characterised by 

 peculiarities of form, though these are less marked than in the cotyledons 

 of Phanerogams. Sometimes the first leaves in Pteridophytes arise laterally 

 on an axis already defined (Equisetum) ; but in other cases, and especially 

 in the megaphyllous forms, the first leaf or cotyledon may appear prior 

 to any definite outgrowth of the axis itself. This fact may be held to be 

 in itself an inherent objection to ranking the cotyledon as the equivalent 

 of a foliage leaf which arises from, the axis; but this objection is met by 

 the fact that free-living leaves, apart from any obvious existent axis, do 

 occur elsewhere in certain specialised cases : these may be interpreted as 

 originating by an anticipatory development, though still in relation to an 

 axis not yet defined by external growth. And so also the cotyledon in the 

 Fern may be held to be essentially an appendage of the axis, the central 

 point of which is already defined in close relation to the intersection of 

 the octant walls of the epibasal segment, but not characterised as yet by 

 external growth : the cotyledon, on the other hand, is hurried forward 

 precociously in its development to meet the physiological need for nutrition, 

 but maintains nevertheless its orientation relatively to the deferred axis. 



