October 21, 1904.] 



SCIENCE. 



53S 



large, connected mth a definite area of ex- 

 tended leaf-base; and still less for tliQ 

 theory that in vascular plants the cortex 

 originated from such coalescent leaf bases. 

 Our theory of the strobilus would, indeed, 

 presuppose that close relation of cortex 

 and appendage, and absence of limit be- 

 tween them, which is so common a feature 

 in vascular plants; and furthermore, it 

 will readily cover the facts where the cor- 

 tex is delimited into definite areas round 

 the leaf bases ; but it does not recognize any 

 necessity for generalizing from such cases 

 of special delimitation that the cortex is 

 foliar in its origin, in shoots of vascular 

 plants at large. It would be more ready 

 to suggest the converse, viz., that the leaves 

 were cortical in their origin, as, indeed, 

 they are in the ontogeny. 



Discussions such as these on phytonic 

 theory, or theory of eortication, are liable 

 to develop into mere scholastic contests. 

 They originated in the present case in the 

 use of terms in an unprecise sense, and the 

 subsequent attempt to attain precision. 

 Both these theories have proceeded from 

 the assumption that the 'leaf is an ab- 

 stract entity, distinct from the stem. Diffi- 

 culties arise when the attempt is made to 

 carry out that distinction sharply in prac- 

 tise, for this is nothing less than the at- 

 tempt to define precisely things which in 

 point of fact appear neither uniform nor 

 precise in nature. The strict definition of 

 terms used in morphological science is 

 doubtless in itself a desirable thing ; but it 

 must be so conducted as to harmonize with 

 the facts of individual development, while 

 at the same time it must not violate evolu- 

 tionary probability. As a matter of fact, 

 neither in the mature state nor in the onto- 

 genetic or phylogenetic development of the 

 leaf does the structure suggest its sharp 

 delimitation from the axis as a general fea- 

 ture in the shoots of ordinary vascular 

 plants. 



My present position with regard to the 

 phytonic theories and the theory of eortica- 

 tion is frankly destructive ; for in the first 

 place, if the evidence from the gametophyte 

 generation be discounted, the facts of 

 segmentation in the sporophyte are of the 

 slenderest; further, I do not think that 

 morphological insight will be advanced by 

 attempts to define precisely the limits 

 of the parts of the vascular shoot; it 

 seems more in accordance with nature 

 to accept for vascular plants the view 

 of Braun and of Sachs, that the shoot is 

 the original unit. What is first urgently 

 required, in order to decide such questions, 

 is the coi-rect recognition of the phyletie 

 lines which eventuated in the various ap- 

 pendages as we see them. Then may fol- 

 low definitions of the parts, which may or 

 may not succeed in assigning their strict 

 limits. When this is accomplished a 

 terminology may follow which shall segre- 

 gate parts which have had a separate 

 phyletie origin. Thus an evolutionary 

 morphology of the shoot would be built up. 

 But it is useless to accept the thesis merely 

 in the abstract that the basis of morphology 

 must be in phylogeny; the principle must 

 also be put in practise, and be ultimately 

 reflected in our methods, and in the defini- 

 tion of our terms. 



A step in this direction will be the recog- 

 nition that at present the word 'leaf is 

 loosely applied; it is, indeed, a temporary 

 makeshift borrowed from colloquial lan- 

 guage, and used in a descriptive rather 

 than in a strictly scientific sense. It des- 

 ignates collectively objects which have, it 

 is true, formal and functional, and even 

 topographical, features in common, but 

 have not had the same phyletie history. 

 There is every probability that the word 

 'leaf will continue to be used in this 

 merely popular sense. 



This position, with its conservative use 

 of terms fitting awkwardly upon advane- 



