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SECTIONAL TRANSACTIONS.—K. 385 
The species occur in colonies having certain features in common, i.e. : 
(1) They occur chiefly in the southern part of Bas-Languedoc, in warm, 
sheltered valleys, and on southern slopes. 
(2) They occur in the early stages of plant succession, and not where the 
climatic climax has developed. 
(3) They are in places difficult of access to man and thus protected from 
destructive effects of cultivation. 
(4) They are, with rare exceptions, on pre-quaternary substrata. 
We know from fossil records that the pliocene flora of the neighbourhood 
was similar to that of the present day, but richer in thermophile species. 
The characteristics of the colonies, and other features of distribution of our 
species, suggest survival. ‘The colonies would appear to represent remnants 
of a more thermophile tertiary flora which, in a few favoured places, has 
been able to survive vicissitudes of climate during the quaternary epoch. 
AFTERNOON. 
Visit to Macaulay Research Institute. 
Saturday, September 8. 
Excursion to Dinnet Moor. 
Sunday, September 9. 
Excursion to St. Cyrus. 
Monday, September 10. 
Joint Discussion with Section D (Zoology, ¢.v.) on Biological problems 
of fresh water (10.0). 
AFTERNOON. 
Prof. J. H. PriestLey.—Vessel differentiation in Angiosperms (2.30). 
The ‘ strip method’ of studying cambial activity makes it possible to 
follow the course of one individual vessel for a comparatively long distance 
in microscopic preparations. A study of vessel differentiation by this 
method directs attention to the rapidity of expansion of the vessel segments 
and of the perforation of the more or less transverse cross walls. ‘These 
processes take place when the wall of the future vessel is very thin. Vessel 
segments have been separated by maceration in this stage, as extremely 
thin-walled elements without signs of pitting. 
By plasmolysis under suitable conditions it has been possible to show 
the presence of protoplasts in the segments of the vessels, after expansion 
and after the cross walls are perforated. In many vessels sheets of pectin 
are present, across the region of perforation, after the cellulose cross walls 
have perforated. © 
The study of vessel differentiation and vessel structure continues to 
emphasise the distinction between ring porous and diffuse porous hardwood 
types. 
