114 ON THE PLANT-CELL. 



intercellular substance to gum. The cells are occasionally observed 

 to exist previous to the formation of these matters ; and the latter are 

 found to increase,, instead of diminish, on the completion of the cellular 

 tissue ; consequently they are in all probability secreted by the cells. 



60. All the cells of the vascular bundles exhibit nearly iden- 

 tical vital processes, and differ for the most part only in their age 

 and the configuration of the walls dependent upon their age. The 

 vessels, when completed, convey air, and perhaps admit juices, but 

 these only occasionally for a short time, and in any case passively. 

 The other elongated cells of the .prosenchyma exhibit, as long as the 

 tissue is living, a rapid change of matter in their interior, and, con- 

 sequently, in general contain a homogeneous watery fluid. They 

 subsequently lose their vitality, and then convey nothing but air. 



That the vessels convey only air, and no juices, may be seen by 

 any one, possessing the least physical knowledge, on the most cursory 

 glance at a longitudinal section of a plant. That any dispute should have 

 arisen on this point, only shows how exceedingly confused most observers 

 are by prejudices and supposed analogies : it is not worth while, however, 

 to waste words about it. It has been already remarked (pp. 57, 64.), that 

 the cells of the vascular bundles probably owe their elongated form itself 

 to a rapid current through them of the sap in a determinate direction, by 

 which means their extremities are more vigorously nourished than their 

 sides. This rapid change explains the circumstance of the chemical 

 processes carried on in them being very simple. We very seldom find 

 peculiar substances formed in them as long as they retain their vitality : 

 even the more solid assimilated matters, as starch, occur in them but 

 seldom, and in small quantity. When they have begun to lose their 

 vitality, however, (to constitute heart -wood,) they for the most part cease 

 altogether to convey sap ; and where they are not completely protected 

 against the external air and moisture, a process of chemical decomposition 

 (decay) is set up, in consequence of which, although retaining their 

 form, they are gradually converted into substances rich in carbon. The 

 peculiar products of the wood, tannin, extractive matter, colouring 

 matter, probably for the most part owe their origin to this process ; less 

 frequently to the sap-channels, bounded by parenchymatous cells, which 

 penetrate the wood, as is the case with the resinous products in the 

 Conifer CB. This subject, however, still presents an extensive field for 

 further investigations. 



61. With regard to the peculiar vital properties of the liber- 

 cells, of the usual form, as seen in the Apocynacea and of the 

 milk- vessels, our knowledge is equivalent to nothing at all. Every 

 thing with respect to these remains to be investigated. 



On the subject of these structures, and especially on the milk- ves- 

 sels, I am rather afraid of saying too much than too little, for, owing 

 to the total neglect of a correct, scientific method, and the puerile sporting 

 with hypotheses, without any foundation or guiding principle, the 

 question respecting them is loaded with such a heap of nonsense, that the 

 best way in beginning upon it is, in the first place, to throw overboard 

 all that has hitherto been done and commence entirely de novo, instead 

 of undertaking the thankless task of cleansing this true Augean stable. 



