PRESIDENTIAL ADDRESS, 707 



develop this fully here, but wide comparison of independent lines of advance 

 suggests that the main types of progress in complexity of the plant-body '" have 

 involved the elaboration of the single filament with apical growth and with 

 subordinated ' branches.' It is generally recognised that various groups of Algee 

 show how a solid multicellular axis may come about not only by the further 

 partition of the segments of the apical cell but by the congenital cortication of a 

 central filament or the congenital condensation of the stibordinated ' branches ' 

 on to the central axis. The Algre further show the change from the dome- 

 shaped apical cell of a filament to the sunken initial cell with two, three, or four 

 sides. The central filament then only appears, if at all, as a subsequent differ- 

 entiation in the tissue, and the segments serially cut off from the apical cell may 

 or may not be^ir projecting hair-shoots or 'leaves.' The Algre thus attain in 

 independent lines a construction corresponding to that of the plant in Liver- 

 worts and Mosses. In the various parallel series of Bryophyta the filament is 

 not only more or less evident in the ontogeny but may be regarded as the form 

 underlying both thallus and shoot, between which on this view there is no 

 fundamental distinction. The sporogonium also can be readily regarded as an 

 elaborated filament. While the same interpretation 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 essential 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 Algae. 



There follows from this a natural explanation of the growth by a single 

 initial cell so commonly found in plants. The apical cell appears to be the one 

 part of the massive plant-body (for instance, of Laiirencia, a moss, or a fern) 

 that persists as a filament ; it is a filament one cell long. It may be replaced by 

 a group of initial cells, as we see in some Algie, Liverworts, and Pteridophyta, 

 and this leads naturally to the small-celled meristems found in most Gymno- 

 sperms and Angiosperms. The filamentous condition is then wholly lost, though 

 the system of relations, and especially the polarity, is maintained throughout all 

 the changes in the apical meristem. 



I feel confirmed in regarding the construction 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 Crypto- 

 gams by Professor Bower." These are (1) the primary importance of the longi- 

 tudinal 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 centre of 

 the epibasal half of the embryo; (3) the probability that embryos without 

 suspen.sors have been derived from forms with suspensors, without any example 

 of the converse change. These and other related facts seem to find their 

 morphological explanation in the slioot of the sporophyte being the result of 

 the elaboration of a filament. 



The Construction of tlio fihoof. 



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 

 subordinated ' branches.' This applies to the multicellular plant-bodies found 

 ill various independent lines of Alg;c and Bryophyta, whether they have definite 

 projecting appendages of the nature of leaves or not. The discarding of the 

 distinction between thallus and .shoot, which in practice has proved an unsatis- 

 factory one, is no great loss. Even taking the word in the narrower sense of 

 ii stem with distinct leaves, the shoots in Alga>, Liverworts, and Mosses, though 



" There are other lines of progression in complexity, the most important 

 being the formation of a plant-body by the co-ordinated growth of a number of 

 filaments. This line of advance is seen in many Algse, in the larger Fungi, and 

 in the Lichens. Though it raises morphological questions of great interest, it 

 is here left out of consideration as not bearing directly on the organisation of 

 the higher plants. 



" Land Flora, chap. xlii. 



z z 2 



