CIRCULATION AND FORMATION OF WOOD IN PLANTS. 571 



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 perme- 

 able, but that, becoming porous as they grow old, they allow the 

 liquids they carry to escape with increasing facility ; and hence a 

 possible interpretation of the fact that, in the older parts, the stain- 

 ing of the tissue around the vessels is so rapid as to suggest that the 

 dye has ascended directly 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 evidence 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 are 

 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 age, 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 herbs 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. And then in very succulent plants, 

 such as Bryophyllum calycinum, Kalancho'e rotundifolia, the various 

 species of Crassula, Cotyledon, Kleinia, and others of like habit, the 

 ducts of old and youngleaves alike retain the dye very persistently : 

 the concomitant in these cases being the small amount of prosen- 

 chyma around the ducts, or the small amount of deposit in it, or 

 both. More conclusive yet is the evidence which meets us when we 

 turn from very succulent leaves to very succulent axes. The tender 

 young shoots of Kleinia ante-euphorbium, or Euphorbia Mauritanica, 

 which for many inches of their lengths have scarcely any ligneous 

 fibres, show us scarcely any escape of the coloured liquid from the 

 vessels of the medullary sheath. So, too, is it with Stapelia 

 Bufonia, a plant of another order, having soft swollen axes. And 

 then we have a repetition of the like connexion of facts throughout 

 the Cactacea; : the most succulent showing us the smallest perme- 

 ability of the vessels. In two species of Rhipsalis, in two species of 

 Cereus, and in two species of Mammillaria, which I have tried, I 

 have found this so. Mammillaria gracilis may be named as ex- 

 emplifying the relation under its extreme form. Into one of these 

 small spheroidal masses, the dye ascends through the large bundles 



