408 MR. SPENCER ON CIRCULATION AND 
of shoots, where the vessels are but incompletely formed out of irregularly joined fibrous 
cells which still retain their original shapes, the dye runs up the incipient vessels and 
does not colour in the smallest degree the surrounding tissue. 
Experiments with leaves bring out parallel facts. On placing in a dye a petiole 
of an adult leaf of a tree, and putting it before the fire to accelerate evaporation, the dye 
will be found to ascend the midrib and veins at various rates, up even to a foot per hour. 
At first it is confined to the vessels; but by the time it has reached the point of the leaf, 
it will commonly be seen that at the lower part it has diffused itself into the sheaths of 
the vessels. In a quite young leaf from the same shoot, we find a much more rigorous 
restriction of the dye to the vessels. On making oblique sections of its petiole, mid- 
rib, and veins, the vessels have the appearance of groups of sharply defined coloured 
rods imbedded in the green prosenchyma; and this marked contrast continues with 
scarcely an appreciable change after plenty of time has been allowed for exudation. 
The facts thus grouped and thus contrasted seem, at first sight, to imply that while 
they are young the coats of these ramifying canals lined with spiral or allied structures 
are not readily permeable, but that becoming porous as they grow old they allow the 
liquids they carry to escape with increasing facility; and hence a possible interpreta- 
- tion of the fact that, in the older parts, the staining of the tissue around the vessels is 80 
rapid as to suggest that the dye has ascended direetly through this tissue, whereas in 
the younger parts the reverse appearance necessitates the reverse conclusion. But now, 
is this difference determined by difference of age, or is it otherwise determined? The evi 
dence as presented in ordinary stems and leaves shows us that the parts of the vascular 
system at which there is a rapid escape of dye are not simply older parts, but ar 
parts where a deposit of woody matter is taking place. Is it, then, that the increasing 
permeability of the ducts, instead of being directly associated with their increasing 28^ 
is directly associated with the increasing deposit of dense substance around them ? 
To get proof that this last connexion is the true one, we have but to take a class of 
cases in which wood is formed only to a small extent. In such cases experiments show US 
a far more general and continued limitation of the dye to the vessels. Ordinary her 
and vegetables, when contrasted with shrubs and trees, illustrate this; as instance the 
petioles of Celery, or of the common Dock, and the leaves of Cabbages or Turnips. N 
then in very succulent plants, such as Bryophyllum calycinum, Kalanchoë rotundifolis 
the various species of Orassula, Cotyledon, Kleinia, and others of like habit, the ducts 
old and young leaves alike retain the dye very persistently, the concomitant in Él 
cases being the small amount of prosenchyma around the ducts, or the small amou? 
of deposit in it, or both. More conclusive yet is the evidence which meets us when V^ 
Sun from very succulent leaves to very succulent axes. The tender your 
Kleinia ante-euphorbium, or Euphorbia Mauritanica, which for many inches 
lengths have scarcely any ligneous fibres, show us scarcely any escape of the col i» 
liquid from the vessels of the medullary sheath. So, too, is it with Stapelia paf" 
p care of another order, having soft swollen axes. And then we have à repetition ” 
pe e : f facts throughout the Cuctaceæ, the most succulent showing > of 
| permeability of the vessels. In two species of Rhipsalis, in two spe? 
