10 



are, however, in my opinion, not merely reservoirs of the nu- 

 tritive substances, but the nutritive substances are formed in 

 them, and then proceed from them through all the forma- 

 tions. 



M. A. Poiteau* has also published some observations on 

 the efflux of the sap from the divided stem of scandent plants, 

 which may be entirely explained in the present state of the 

 science. He cut the stem of a climber right through, and no- 

 ticed that at neither of the two surfaces of section did water 

 exude; if, however, apiece of the stem, about 4 feet in length, 

 was cut off, the water contained in it flowed out immediately f. 

 I have already frequently remarked that this water is contained 

 in the metamorphosed spiral vessels, which in the stems of 

 climbers, as in the vine, are so large that they can no longer act 

 as capillary tubes, and therefore the efflux of the sap from the 

 divided ends of the stem of such plants is entirely subject 

 to the pressure of the atmosphere ; it takes place instanta- 

 neously if the end of the cut stem is placed vertically, on the 

 contrary, very slowly if this lies in a horizontal position. If, on 

 the other hand, we separate the upper part of the stem, which 

 is still clothed with its leaves, from the root end, no water 

 can flow* out of the end of the first portion, because the trans- 

 piration of the leaves, when their number is sufficiently great, 

 developes a force by which the water is retained in the large 

 spiral vessels. This fact has been proved by numerous expe- 

 riments in the second part of my Vegetable Physiology. The 

 reason still remains to be explained why no water proceeded 

 from the surface of the section of the lower end of the stem 

 of the climber, as M. Poiteau observed to be the case. It has 

 been proved by numerous observations, that the tears of vege- 

 tables, i.e. the efflux of their crude nutritive sap above the 

 level of the wounds which are inflicted on the stems of such 

 plants, are to be ascribed entirely to the endosmosis of the 

 fibrils, and that this phenomenon only takes place if the plants, 

 as during the production of leaves and buds, require a large 

 quantity of nutriment. 



On the circulation of the sap in Chara a paper by M. Du- 



* Note sur la Liane des voyageurs. Ann. des Sciences Naturelles, Avril 

 1837, p. 233. 



f Lond. and Edin. Philosophical Magazine, vol. xi. p. 525. 



