196 Comparative Physiolog]). 



2. Woody, or Fibrous, Tissue; 3. Vascular Tissue; but has the 

 new addition of, 4. Laticiferous Tissue. " The basis of all the 

 elementary tissues may be considered as membrane and^Jre; 

 the one, perhaps, formed by the adhesion of single particles in 

 expanded surfaces, the other by their union in lines." Some 

 have thought them produced by the different kinds of elec- 

 tricity; that which gives out the brush producing membrane, 

 while that which gives off the electrical matter in a pointed 

 form produces fibre. Vegetable membrane he defines as per- 

 meable to fluids, though always, unless in some very few 

 instances, destitute of visible pores. Elementary fibre he com- 

 pares to hair of extreme tenuity, often not exceeding yg^oo 

 of an inch, usually disposed in a spiral direction ; the adjacent 

 threads having a peculiar tendency to unite and grow together ; 

 whether hollow or solid not easily determined. The descrip- 

 tions of the tissues are similar to those of other authors. Cel- 

 lular tissue, varying from J^ to g qVo ^^ ^^ inch in the diameter 

 of the cells, is capable of growth in all directions, forming 

 the parenchyma, or flesh of plants, and the great bulk of the 

 organs in which active vital processes are performed. A 

 modification of this (the elongated cellules of De Candolle and 

 others) he describes as vasiform tissue, the largest of all kinds 

 of tissue formed by the union of cells laid end to end, the par- 

 titions between them being more or less obliterated, thus forming 

 a continuous tube. The descriptions of ligneous tissue, or 

 woody fibre, forming the essential organs of support, consti- 

 tuting first the alburnum or sap wood, and afterwards, by the 

 deposition of various secretions in its tube, forming the du- 

 ramen or heart wood, with its modifications in the Conifer^e, 

 or fir trees ; as also of the vascular tissue, distinguished by pos- 

 sessing a spiral fibre coiling within its membranous tubes from 

 end to end, resembling the tracheae or air vessels of insects in 

 always containing air, though, being closed vessels, the gaseous 

 contents have to permeate the delicate membrane of the tubes 

 in plants closed at the end. These are described in a manner 

 so similar to other elementary works as to prevent the necessity 

 of lengthening out the present essay by any more detailed 

 account. The laticiferous tissue he describes " aa consist- 

 ing of a series of branching tubes anastomosing with each 

 other, so as to form a network, in which the tatex, or elaborated 

 sap, flows. This branching character is its chief difference from 

 other forms of tissue ; the walls of the vessels, though very thin 

 and scarcely visible in the young plant, become thickened after- 

 wards, but the structure is not different from what we find 

 elsewhere. The sides are not parallel as in other vessels, but 

 often contracted and expanded at intervals ; the average dia- 

 meter about Y4^oo of an inch. This tissue is present in most 



