TRANSACTIONS OF SECTION K. 911 



The roots Are found to be contracted to about half their original length. Later on 

 they are cut off by layers ol" periderm from the tuber, just as a dead leaf becomes 

 detached from its stem. 



If one of these tubers is replaced at the surface of the soil it sends out a ne\V 

 set of contractile roots and is again pulled down to its normal depth of about 

 6 inches. 



3. Evaporation in relation to the Stratification of Marsh Vegetation. 

 By Professor R. H. Yapp, M'.A. 



The evaporating power of the air may be considered as an approximate 

 measure of the pull exerted by the atmosphere on the water contained in leaves 

 and other transpiring organs of plants. 



Evaporation readings were taken at different levels in and above tbe marsh 

 vegetation of Wicken Fen, under varying atmospheric conditions. It was shown 

 that evaporation varies according to the density of the vegetation and the depth 

 of the stratum in winch the readings are taken. For example, in the case of a 

 mixed vegetation of mar.^h plants, about 2 feet in height, the average evapora- 

 tion in tbe free air above is about 1'7 times that in the layer immediately below 

 the top of tbe vegetation, and 6'8 times that in a stratum 18 inches below the 

 surface of the vegetation. 



4, Report on the Registration of Botanical Photographs. 



5. Report on the Structure of Fossil Plants. — See Reports, p. 493, 



6. Rtiport on Studies of Marsh Vegetaiion>^Bee Reports, p. 492. 



7. Intf.rim Report on the Sequence of Plant Remains in the Peat 

 Deposits qf Teesdale, <&c.— -See Reports, p. 493. 



8. Seco-iid IntisHM Report on Research on South African Cycads and on 

 Welioitschia. — See Reports, p. 494. 



9i A Plant Animal. By Professor F. W. Keeble, 



Tt/ESDAY, SEPTEMSEH 8. 

 Th'6 following Papers were read : — 



1. The Origin of Dicotyledons. By W. C; Worsdell, 



The various existent plant-pLyla have a common origin in the remote past. 

 The similar structures met with in each group are therelbre analogical, and not 

 lioraological in character, and must always represent inetancea of parallel develop- 

 taent along distinct lines. 



