327 
1857, page 434, which, so far as I can judge, is a most able exposition. 
The author expresses his belief in a double circulation, or, in other 
words, a rise and descent of the sap, which he calls the great circulation, 
and also in a secondary or smaller circulation through the lactiferous 
vessels, which he calls the venous circulation. He further argues against 
the probability, amounting almost to impossibility, of the physical ac- 
tions of endosmose and capillarity playing so important a part in sap 
circulation as they have been so generally believed to do by physiologists. 
But what chiefly concerns my present subject is, his account of the 
manner descending sap acts in forming the tissues which constitute 
wood. To quote a paragraph from M. Trecul’s paper will enable me to 
make his views on the matter better understood than anything I can 
state, viz. :—‘‘The sap, which onits way takes part in the nutrition of 
the organs first developed, arrives in the leaves, in the green paren- 
chyma of which it is submitted to fresh elaboration, or in the chlorophyl 
cells of the stems of fleshy plants without leaves. The carbonic acid of 
the air is absorbed, and then decomposed during the day ; its carbon is 
retained by the sap, and its oxygen is in great part rejected. The sap, 
thus modified under the influence of respiration, takes its course through 
the cortical cells, which it nourishes. It then aids in the multiplication 
of the cells of the generative layer, which are produced in horizontal 
series. A portion of these cells, thus horizontally multiplied, forms a 
new layer of bark, the woody fibres and medullary rays ; the others are 
converted into vessels in the following manner—the excess of the de- 
scending sap, which is not employed in the 
nutrition of the newly formed cells, or in 
thickening those first developed, descends 
through certain of the newly-formed cells, 
it dilates them, perforates them, and makes 
them take all the characters of vessels, so 
that those cells which, during the first phase 
of their developement, resembled all the 
others, appear subsequently to be of a totally 
different nature.’’ Now this appears to me to 
be a succinct statement of the true theory of 
the formation of vascular tissues in exoge- | 
nous plants, one which all my experiments \} 
agree with, and that on which the circum- ‘ 
stance of stocks on which plants are grafted 
not being covered with layers of the wood 
of the graft united to them, can be easily de- 
monstrated. 
The specimens agreeing with diagram 
No. 3 were denuded of their bark, and had 
also the cambium layer scraped off; yet they 
have continued to grow during the last six 
years, and the parts denuded have increased 
slightly in circumference, though much slower than the portions, on 
R. I. A, PROC.—VOL. VI. 3A 
