MASS MOVEMENTS OF PLASTIC MATTERS. 361 



since there is no passage for it to escape. In the neighbourhood of the growing buds, 

 however, the tension of the tissue is much smaller, and the pressure of the paren- 

 chyma on the young sieve-tubes there situated feebler, whence it follows that even 

 in the intact plant the contents of the sieve-tubes are forced towards the buds by the 

 pressure of the parenchyma of older parts of the shoot, and this, in accordance with 

 growth, need only take place slowly. It is not impossible that yet more special 

 relations of organisation co-operate here ; and the emptying of the sieve-tubes, or, 

 better, the dilution of their contents by water in older shoot-axes, and perhaps also 

 the formation of the Callus" (in trees, especially in the autumn), indicates something 

 of the kind. Nevertheless, scarcely anything definite can at present be said in this 

 connection. 



With respect to the pressure of the surrounding parenchymatous tissues, the 

 laticiferous vessels ', where they exist, are in the same position as the sieve-tubes. 

 The simple fact that every wounding of a fresh turgescent plant which contains latex 

 causes the immediate production of a thick drop of latex, and that in old, large 

 specimens of succulent Euphorbias even streams of latex exude, and in a few 

 seconds yield several cubic centimetres of liquid, alone suffices to demonstrate that the 

 Jatex is extruded from wounds with great force. That this is not in any way a mere 

 outflow, follows at once from the fact that after cutting across a milky stem, not only 

 -the lower cut surface of the apical portion but the upper one of the root-stock also 

 extrudes the latex. Besides, the laticiferous vessels are extremely narrow capillary 

 tubes, the normal terminations of which in the buds, leaves, and root-apices are 

 closed : how could fluid flow out at all on cutting such capillaries closed at the 

 ends? It would scarcely have been worth while to introduce these self-evident 

 reflections, were it not for the fact that even celebrated botanists have doubted the 

 action of the pressure which I rendered evident so long ago as 1865 (Experimental- 

 physiologie, p. 386). The laticiferous vessels as well as the sieve-tubes behave in this 

 connection exactly like the blood-vessels in the human body : when we wound our- 

 selves the blood does not simply flow out, it is driven out. 



On now enquiring what significance this pressure, exerted from all sides on the 

 laticiferous vessels, may possibly have in the economy of the plant, it is to be noticed 

 that the pressure of the tissues diminishes in the neighbourhood of the growing buds 

 and other young organs, and that therefore the stronger pressure of the parenchyma 

 in the older succulent parts must drive the latex towards the younger growing parts ; 

 though this, of course, cannot be regarded as exhausting the mechanics of the subject. 

 Nevertheless, we know at present no more than what has been said. However, 

 that a continuous movement of latex from the older and especially the assimilating 

 organs to the young growing parts also comes into consideration for the supply 

 of plastic substances, follows from the fact, confirmed by numerous observers, that 

 larger or smaller quantities of proteid substances, fats, and carbo-hydrates (in the 



' After Schultz-Schultzenstein had previously ascribed to the latex an unduly high significance 

 for the life of the plant, and had at the same time connected with it the most confused errors, in a 

 prize essay rewarded by the Parisian Academy, the importance of the latex was again far under- 

 estimated by Hugo von Mohl in a critical reply. I led the way for the correct appreciation of the 

 significance of latex and its receptaicles for the nutrition of the plant in my 'Experimental- 

 physiologie'- (1865, p. 386). 



