October 21, 1904.] 



SCIENCE. 



529 



and lie concludes that 'we may [therefore] 

 say with perfect justice that the plant 

 * * * is, as regards form, essentially a 

 system of axes.' Naegeli contemplated a 

 somewhat similar origin of the leafy shoot, 

 as an alternative possibility; in fact, that 

 the apex of a sporogonium-like body elon- 

 gated directly into that of the leafy stem, 

 in which case the axis would be the per- 

 sistent and prominent part, and the leaves 

 be from the first subsidiary, and lateral ap- 

 pendages. In my theory of the strobilus 

 in archegoniate plants the central idea was 

 somewhat similar to this. It may be briefly 

 stated thus: There seems good reason to 

 hold that a body of radial construction, 

 having distinction of apex and base, and 

 localized apical growth as its leading char- 

 acters, existed prior to the development of 

 lateral appendages in the sporophyte; for 

 such a body is seen in certain bryophyte 

 sporogonia, while the prior existence of the 

 axis and lateral origin of the appendages 

 upon it are general for normal leafy shoots. 

 The view thus put forward is, indeed, the 

 mere reading of the story of the evolution 

 of leaves in terms of their normal indi- 

 vidual development. I have recently 

 shown that all pteridophyte shoots may be 

 regarded as derivatives from the radial 

 strobiloid type, with relatively small 

 leaves, which would thus have come into 

 existence. 



It is natural to look to the pteridophytes 

 for guidance as to the origin of foliar de- 

 velopment in the sporophyte, for they are 

 the most primitive plants with leafy sporo- 

 phytes. They may be disposed according 

 to the prevalent size of their leaves in a 

 series, leading from microphyllous to mega- 

 phyllous types. I have lately shown that 

 such a seriation is not according to one 

 feature only, but that certain other char- 

 acters which have been summarized as 

 ' filicineous ' tend to follow with the in- 

 creasing prominence of the leaf. This 



indicates that such seriation is a natural 

 arrangement. Now it is possible to hold 

 either that the large-leaved fern-like plants 

 were the more primitive, and the smaller- 

 leaved, derivatives from them by reduc- 

 tion ; or, conversely, that the smaller-leaved 

 were the more primitive, and the larger- 

 leaved derivatives from them by. leaf-en- 

 largement; other alternative opinions are 

 also possible, such as that the leaf origin 

 has been divergent from some middle type, 

 or that the leaves of vascular plants may 

 have been of polyphyletic origin.* For 

 the moment we shall leave these latter 

 alternatives aside. 



Much of the difference of view as to 

 foliar origin centers round the question 

 whether originally the leaf was relatively 

 large or small. Those who hold that the 

 large-leaved forms were the more primitive 

 will be naturally disposed towards the view 

 of the original preponderance of the leaf 



* The view recently advanced by Professor 

 Lignier ( ' Equisetales, et Sphenophyllales. Leur 

 origine filioinienne commune,' Bull. 8oc. Linn, de 

 Worviandie, Serie 5, Vol. 7, Caen, 1903) is analo- 

 gous to that of Potonig, though differing from it in 

 detail. It involves the ranking of the lycopod leaf 

 as a ' phylloid,' the leaf of the fern as a true leiaf, 

 or ' phyllome,' differentiated from an indifferent 

 system of ' cauloids,' on which the ' phylloid ' ap- 

 pendages had become abortive. It regards the 

 leaves of equiseta and sphenophylls as phyllomes, 

 reduced from the larger-leaved fern-type. The 

 argument is chiefly based on comparisons as to 

 branching and anatomical structure. I do not 

 think that these grounds suffice to override the 

 probability that the leaves of lyeopods are es- 

 sentially of the same nature as those of the 

 sphenophylls or equiseta (compare my ' Studies,' 

 No. v.). Professor Lignier's view further in- 

 volves the acceptance of homologous alternation, 

 while he makes no mention of the chromosome- 

 differences of the two generations. Such diffi- 

 culties do not arise if the leaves of the spheno- 

 phylls and equiseta are regarded as being in the 

 upward rather than the downward scale of de- 

 velopment, a view of them which would equally 

 harmonize with the anatomical comparisons of 

 Professor Lignier. 



