184 



THEORY OF RECAPITULATION 



leaf-tip which supplies the apex of the new bud, but from a position near 

 it upon the convex side of the mother-leaf. As Goebel remarks, this finds 

 its parallel in the formation of the embryo and in the apogamous origin of 

 a Fern-plant on a prothallus. Through such examples we arrive at a con- 

 ception of a leaf also as a part which may be at times of independent 

 origin, and not necessarily produced from a pre-existent and obvious axis. 

 But the cases above quoted from mature plants are almost certainly 

 secondary, and are probably consequent upon peculiar conditions of life. 

 The question then presents itself whether the independent origin of a leaf as 

 it is seen to occur in certain embryos is not also a secondary condition in 

 descent, and a consequence of what might be called anticipatory develop- 

 ment of that part of the shoot, to meet such early biological needs as that 

 of assimilation or of storage? It is impossible to answer such a question 

 with any approach to proof: nevertheless the case of A. Edgworthi is 



very suggestive of such a 

 detached and anticipatory 

 development of an indivi- 

 dual part. Clearly the early 

 appearance of a leaf in the 

 Fern-embryo would be an 

 advantage, while the axis 

 is in no way essential for 

 the performance of its first 

 functions. If such be the 

 origin of the first leaf or 

 leaves of a sporophyte em- 

 bryo, then so far from their 

 independent position being, 

 as is usually assumed, the 

 primitive position, it would 

 be secondary, a mere result 

 of adaptation to the early 

 requirements of the em- 

 bryo. This question will 

 be specially studied later 

 in connection with the 



embryogeny of the Lycopods, a family in which the diversity of character 

 of the prothallus has imposed considerable and instructive differences of 

 development upon the embryo. Meanwhile I see no sufficient reason, on 

 the ground of their position or the mode of their origin, to regard the 

 "cotyledon" or "protophyll" as representing a category essentially apart 

 from foliage leaves : l nor does the apparently independent existence of 



1 Goebel, Organography, ii., p. 400, remarks specifically for Pteridophytes Unit the 

 cotyledons "are without exception arrested forms of foliage leaves": he extends the 

 conclusion also to Seed-plants (p. 402). 



; FIG. 94. 



Adiantutn Edgworthi. Origin of leaf borne buds. I. = apex of 

 leaf seen from above : the apical cell has divided by a cross- 

 wall. X position at which the first leaf of the bud arises, /^posi- 

 tion of origin of the lateral leaf-series whence usually in a leaf the 

 pinnules develop. II. Apex of leaf seen from the side, lettering the 

 same. III. Apex of leaf in optical longitudinal section : j = divided 

 apical cell; / = first leaf of the bud. IV. Somewhat older stage. 

 V. Apex of leaf in longitudinal section : = apex of bud surrounded 

 by scales ; A = first leaf, looking like the continuation of the mother- 

 leaf; / = incipient root. I. -I V. highly magnified. V. less highly 

 magnified. 



