TRANSACTIONS OF SECTION K. S2o 



cambium enabled tliem to provide for continued additions to their carrying system. 

 Where such monostely and secondary growth occurred in these older types their 

 adaptation in these respects to water-carriage was on lines similar to those of our 

 dominant Dicotyledones and was effective in giving them dominance in their 

 epoch. There is no more interesting page in the history of evolution than that — 

 and we owe it in large measure to the labours of Scott and Seward — upon which 

 is depicted the struggle of some polystelic forms amongst these old plants to 

 achieve the structural facilities more easily attained through monostelic construc- 

 tion. The existence of polystely in a few Angiosperms only confirms the advantage 

 which the whole group has derived from its monostely. Such polystelic forms 

 amongst them as we know have many of them special water-adaptations, and in 

 no case can they be said to be progressive types. 



I do not need to remind you that vasa are not the exclusive possession of the 

 angiospermous type, but they are the conspicuous feature of their carrying system, 

 whilst the tracheid is the leading one in the older type of vegetation. All ana- 

 tomical evidence indicates tbat vasa give greater facility to rapid transport 

 of water than do other elements, and we may, therefore, conclude that they have 

 been adjuvants in enabling the Angiosperm to meet effectively the demand made 

 upon it by the drier atmospheric conditions. 



I now pass on to consider from the same standpoint the classes which make up 

 tlie group of Angiosperms. 



Of the Classes of Angiosiierms. 



There has been for long a general recognition of two classes amongst the 

 Angiosperms — Dicotyledones and Monocotyledones — separated one from the other 

 by definitive characters which I need not specially depict here. Recently, how- 

 ever, we have seen an attempt made by Van Tieghem to establish another clasps — 

 that of Liorbizal Dicotyledones — for which is claimed a rank equal to that of the 

 Dicotyledones and Monocotyledones. Were this valid it would be a matter of 

 supreme importance, for whatever be the relationship between Dicotyledones and 

 Monocotyledones there can be no doubt of their having developed as distinct 

 groups within the whole period of which we have knowledge of them, and the 

 existence of a third class intermediate or outside of them might lead to interesting 

 conclusions. It is worth while, therefore, to consider the evidence on which this 

 class is founded. It includes two of our recognised families — the Nymphreacese 

 and the Graminese. 



What is the exact position and the affinities df the Nymphseacese amongst 

 Angiosperms is no new theme of discussion. That they have characters resem- 

 bling those of Monocotyledones ^ has been often insisted on. Van Tieghem 

 lays stress on what he considers the monocotylous differentiation of the root-apex 

 and the derivation of the piliferous layer from the same meristem-initials as the 

 cortex, whilst in the embryo he finds the two cotyledons of Dicotyledones. But 

 the most recent observations of the embryogeny of the family go to show that the 

 embryo is that of the monocotylous plants, the apparent dicotylous character 

 being the result of the splitting of one cotyledon. If this be so the position of 

 Nymphfeacese will be amongst the Monocotyledones, a position the root-characters 

 in Van Tieghems view will support. But whether this be confirmed by further 

 research or no— and a complete reinvestigation of their embryogeny and develop- 

 ment is much wanted — what we may say at present is that it is not in features 

 such as this one of the root-apex — which is, after all, not so simple and uniform 

 as ^'an Tieghem would have it — that we are likely to find phyletic diagnostic 

 characters of groups. 



The reason for the inclusion of the Graminere in this new group is the 

 assumed presence of a second cotyledon. The construction o f the embryo of grasses 

 is peculiar, as is well known, and has for a long time been a main support of the 



' The anatomical characters upon which this resemblance was chiefly based are 

 now known to be of another nature. 



