518 OKGANOLOGY. 



our climate. At all other seasons, and in other plants, endosmosis is 

 assisted by evaporation through the leaves, and it is very probable that the 

 passage of sap at these seasons of evaporation is stronger and quicker 

 than that in the spring. Although much has been said on the subject, yet 

 useful observations fail us. With regard to tropical plants, cultivated 

 artificially in hothouses, they do not offer information of a kind that can 

 be safely relied on. Many twining plants of the tropics, when cut 

 through, allow much sap to exude, and Meyen therefore thinks that they 

 are always to be found in the condition of our forest trees in the spring. 

 I think that such an opinion is without foundation, and I wish that our 

 governments, instead of sending out mere collectors of species, would 

 send out some with the necessary authority and proper instruments 

 into those countries where these phenomena are to be observed. 



The second question is concerning the course of the sap in plants. 

 The facts are as follow: The so-called vessels in most plants never 

 convey sap ; and with others it is probable that they convey it only 

 during a few weeks while the new buds are forming : where the greatest 

 consumption of sap is going on, the vessels of the part are not found to 

 contain it. In many important organs, where the vital processes of 

 vegetation go on, and formative energy is present, as, for example, in 

 the stamens and ovules, there are scarcely any vessels : large masses of 

 parenchyma, in which thousands of cells lie close together, actively 

 vegetating, contain no vessels : five great families of plants, namely, 

 Algae, Lichens, Fungi, Mosses, and Liverworts, have no trace of vessels ; 

 and amongst other plants several species have no vessels. After such 

 premises, unprejudiced observers will hardly assume the motion of the 

 sap through the vessels, or draw conclusions upon such a presumption. 

 Nothing is more certain than that in most cases the nutritive fluid which 

 the single cells need, must be taken up from other cells ; and it is super- 

 fluous to imagine another mode of conduct of the sap for less frequent 

 and less important cases. On the significance of the vessels, and the 

 bundles of vessels, I have before spoken ( 34.); and the conditions 

 which they present, their origin and their form, appear to leave no doubt 

 that they are the effect, and not the cause, of a living movement of the 

 sap in a fixed direction. Where there is a considerable formative pro- 

 cess and great chemical activity exhibited, the circumstances of a 

 stronger endosmosis exist, and a greater stream of sap is afforded. This 

 stream of sap acts upon the cells through which it passes agreeably to 

 the laws of cell-life. The cells become changed into lengthened cells and 

 vessels, and thus far allow the passing of the sap. For this reason vas- 

 cular bundles are seen near every bud, and especially the most active 

 developing terminal bud, and also near each developing leaf, &c. Where 

 chemical activity is feeble there is no such active passage of sap, which 

 shows that it exercises an important transforming influence in the cells. 

 The originating cause at work in this case is the attractive power of the 

 mixing heterogeneous fluids ; but the possibility of the motion lies in the 

 universal property of vegetable membrane of allowing fluids to pass, 

 the capacity of imbibition ( 39.). I have already given, in my treatise on 

 the Cacti, my views of this subject, and remarked that we need not seek 

 any further explanation of how the fluid passes through the membranes, 

 but rather why, in certain cases, it is held back. The reason thereof is 

 partly that one; side of the membrane is in contact with air which cannot 

 escape, and which cannot be absorbed by the fluids contained in it, and 

 partly that there are on each side of the membrane fluids which will not 



