SECTIONAL TRANSACTIONS.— K. 385 



Prof. R. H. CoMPTON. — Features of Botanical Interest in the National 

 Botanic Gardens. 



Afternoon. 



Dr. Margery Knight. — Lecture (semi-popular) on Seaweeds, a Study of 

 Adaptation and Opportunity. 



Wednesday, July 24. 



Prof. J. H. Priestley. — The Movement of Water and Solutes in the Tree 

 (in the Department of Forestry, for which see later pages). 



A study of the seasonal activity of the cambium in the tree throws quite a new 

 light upon the processes by which water is moved, in the early part of the growing 

 season, from the trunk into the young shoots, and upon the manner in which organic 

 solutes are transferred downwards in the tree towards the close of the growing season. 



Prof. M. ViCTORiN. — Some Evidence of Evolution in the Flora of North- 

 Eastern America. 



Evidences of evolution are seen in the presence of local endemics in definite parts 

 of North-Eastern America. The origin of these endemics is ascribed to isolation, 

 both geographical and physiological. Geographical isolation has been most effective : 

 (a) by separation, lengthwise, of North America into two separate land masses during 

 Cretaceous times ; (6) by submergence of the North Atlantic land-bridge with Eurasia 

 at the end of Tertiary times ; (c) by glaciation. 



Physiological isolation has produced endemics in North-Eastern America, chiefly 

 in the case of estuarine plants. 



Dr. A. S. Hitchcock. — Grasses in relation to Man. 



Prof. Francis E. Lloyd. — The Mechanism of the Trap of Utricularia. 



It is now well established bj' the work of Merl and of Czaja that the trap of 

 Utricularia is a self-setting mechanism, that the setting results from the expulsion of 

 water by the walls, resulting in reduced pressure within entailing their collapse, and 

 that this condition is normally sustained for long periods, during which no water 

 enters. It is evident that the door or valve must be a very efficient mechanism. As 

 to how this efficiencj' is attained, the prevailing view has been that the door edge 

 rests with its outer aspect very firmly on the threshold (all observers except Brocher 

 and Withj-combe), or pressing outwardly against a ridge assisted by the curvatures 

 of the wails (Brocher), or finally by pressing inwardly against the forwardly directed 

 sides of the glandular cells clothing the upper surface of the threshold (Withycombe) ; 

 but in all cases the explanation postulates the presence of mucilage (mucus) as a sealing 

 substance. 



It will be argued that Withycombe's idea, though not substantiated by his evidence, 

 is correct, but that only the middle portion of the free edge of the door (Czaja) rests 

 against the upper, but forwardlj^ directed, surface of the pad, to the shape of which 

 due attention must be directed. The lateral portions of the door traverse the pad 

 in such fashion that there is a sharp re-entrant angle between door and pad, affording 

 places which could not resist the water pressure without leaking were it not for some 

 adequate means of stanching the flow. There is no mucus or mucilage as previously 

 held. There is, however, a membrane, derived by exfoliation of the cuticle of the 

 glandular cells of the pad, which becomes free inwardly and remains attached outwardly 

 to form a second valve which completely seals the entrance bj' overlapping the door 

 edge and the lateral folds. The extremely sudden operation of the door on touching 

 the trigger hairs, and the water-tight character of the whole, is thus satisfactorily 

 explained. The species studied is U. gibba. 



1929 C C 



