414 MR. SPENCER ON CIRCULATION AND 
axes not liable to be bent backwards and forwards in any considerable degree by the 
wind, has, ramifying through its tissue, vessels that allow but an extremely slow escape 
of dye and have unthickened sheaths. Such of the Euphorbias as have acquired the 
fleshy character while retaining the arborescent growth, like Huphorbia Canariensis, 
teach us the same truth in another way. In them the formation of wood around the 
vessels is inconspicuous where the intermittent strains are but slight; but it is conspi- 
cuous at those joints on which lateral oscillations of the attached branches throw great 
extensions and compressions of tissue. Throughout the Cactacee we find varied ex- 
amples of the alleged relation. Mammillaria furnishes a very marked one. The sub. 
stance of one of these globular masses, resting on the ground, admits of no bending from 
side to side; and accordingly its large bundles of spiral and annular vessels, or partially 
united cells, have very feebly-marked sheaths not at all thickened. In such types as 
Cereus and Opuntia we see, as in the Euphorbias, that where little stress falls on the 
vessels, little deposit takes place around them ; while there is much deposit where there 
is much stress. Here let me add a confirmation obtained since writing the above. After 
observing among the Cactuses the very manifest relation between strain and the forma- 
tion of wood, I inquired of Mr. Croucher, the intelligent foreman of the Cactus-house aí 
Kew, whether he found this relation a constant one. He replied that he did, and that 
he had frequently tested it by artificially subjecting parts of them to strains. Neglect- 
ing at the time to inquire how he had done this, it afterwards occurred to me that if he 
had so done it as to cause constant strains, the observed result would not tell in favour 
of the foregoing interpretation. Subsequently, however, I learned that he had produced 
the strains by placing the plants in inclined attitudes—a method which, by permitting 
oscillations of the strained joints, allowed the strains to intermit. And then, making 
the proof conclusive, Mr. Croucher volunteered the statement that where he had pro 
duced constant strains by tying, ro formation of wood took place. 
Aberrant growths of another class display the same relations of phenomena. Take 
first the underground stems, such as the Potato and the Artichoke. The vessels which 
run through these, slowly take up the dye without letting it pass to any considerable 
extent into the surrounding tissues*. Only after an interval of many hours does the 
prosenchyma become stained in some places. Here, as before, an absence of rap! 
exudation aecompanies an absence of woody deposit; and both these go along with the 
absence of intermittent strains. Take again the fleshy roots. The Turnip, the Carrot, 
and the Beetroot have vessels that retain very persistently the coloured liquids they 
take up. And differing in this, as these roots do, from ordinary roots, we see that they 
also differ from them in not being woody, and in not being appreciably subject to the 
usual mechanical actions. In these cases, as in the others, parts that ordinarily becom 
dense, deviate from this typical character when they are not exposed to those forces 
which produce dense tissue by increasing the extravasation of sap. 
* 
* Those who repeat these experiments must be prepared for great irregularities in the rates of absorp | 
eulent structures in general absorb much more slowly than others, and sometimes will scarcely take up the dye * 
— differences between different structures, and the same structure at different times, probably depend on the degre" 
in which the tissues are charged with liquid and the rates at which they are losing it by evaporation. 
tion. Sue 
