December 3, 1915] 



SCIENCE 



787 



or ' ' leaves. ' ' The algse thus attain in inde- 

 pendent lines a construction corresponding 

 to that of the plant in liverworts and 

 mosses. In the various parallel series of 

 Bryophyta the filament is not only more or 

 less evident in the ontogeny, but may be re- 

 garded as the form underlying both thallus 

 and shoot, between which on this view there 

 is no fundamental distinction. The sporo- 

 gonium also can be readily regarded as an 

 elaborated filament. While the same inter- 

 pretation of the fern-prothallus will readily 

 be granted, to think of the fern-plant as 

 the equivalent of an elaborated filament 

 may appear far-fetched. So far from this 

 being the case, I believe that it will be 

 found helpful in understanding the essen- 

 tial morphology of the shoot. In a number 

 of vascular cryptogams and seed-plants, 

 there is actually a filamentous juvenile 

 stage, the suspensor, while the growth by 

 a single apical cell is essentially the same 

 in the fern as in the moss and some algas. 



There follows from this a natural expla- 

 nation of the growth by a single initial cell 

 so commonly found in plants. The apical 

 cell appears to be the one part of the mass- 

 ive plant-body (for instance, of Laurencia, 

 a moss, or a fern) that persists as a fila- 

 ment ; it is a filament one cell long. It may 

 be replaced by a group of initial cells, as 

 we see in some algffi, liverworts and Pteri- 

 dophyta, and this leads naturally to the 

 small-celled meristems found in most 

 Gymnosperms and Angiosperms. The fila- 

 mentous condition is then wholly lost, 

 though the system of relations and espe- 

 cially the polarity is maintained throughout 

 all the changes in the apical meristem. 



I feel confirmed in regarding the con- 

 struction of the sporophyte in this fashion 

 by the fact that it fits naturally with the 

 conclusions resulting from the masterly 

 comparative treatment of the embryology 

 of the vascular cryptogams by Professor 



Bower. These are (1) the primary impor- 

 tance of the longitudinal axis of the shoot, 

 the position of the first root and the foot 

 being variable; (2) the constancy of the 

 position of the stem-apex near the center of 

 the epibasal half of the embryo; (3) the 

 probability that embryos without suspensors 

 have been derived from forms with sus- 

 pensors, without any example of the con- 

 verse change. These and other related facts 

 seem to find their morphological explana- 

 tion in the shoot of the sporophyte being the 

 result of the elaboration of a filament. 



The view to which we are thus led is 

 that the uniaxial shoot is a complex whole, 

 equivalent to the axial filament together 

 with its congenitally associated subordi- 

 nated "branches." This applies to the 

 multicellular plant-bodies found in various 

 independent lines of algse and Bryophyta, 

 whether they have definite projecting ap- 

 pendages of the nature of leaves or not. 

 The discarding of the distinction between 

 thallus and shoot, which in practise has 

 proved an unsatisfactory one, is no great 

 loss. Even taking the word in the nar- 

 rower sense of a stem with distinct leaves, 

 the shoots in algse, liverworts, and mosses, 

 though admittedly independent develop- 

 ments, exhibit an essential correspondence 

 amounting to a homology of organization. 

 The resemblances are not analogies, for it 

 is doubtful whether the "leaves" in the 

 different cases correspond in function. 

 The comparison of the shoot of the sporo- 

 phyte of a vascular cryptogam with, for 

 example, the shoot of the moss seems 

 equally justifiable. It is only forbidden 

 by strict phyletic morphology, which for 

 our purpose has no jurisdiction. The gen- 

 eral agreement as regards the leaf-arrange- 

 ment between the ferns and the Bryophyta 

 suggests that similar laws will be found 

 to hold in the shoot of both gametophyte 

 and sporophyte. Apart from plagiotropic 



