IX] SPONGY TISSUE 89 



in the lower moiety of the section. Here the cells them- 

 selves are irregular in shape, and only in close contact one 

 with the other, or with the epidermis of the lower surface 

 of the leaf, by relatively small parts of their surfaces : 

 they have been driven apart by the formation of large 

 cavities, or intercellular spaces, as they are called, so 

 that the whole tissue presents a close resemblance to a 

 sponge, and so this portion of the mesophyll is termed 

 the spongy tissue (Fig. 26). 



The leaves of all our common trees and shrubs, such 

 as Clematis, Barberry, Lime, Horse-chestnut, Maples, 

 Dogwood, Rhamnus, Robinia, Laburnum, Primus, Pyrus, 

 Roses and Brambles, Ribes, Honeysuckles, Ash, Elms, 

 Alder, Birch, Hornbeam, Hazel, Walnut, Plane, Beech, 

 Oaks, Poplars and Willows, agree in every essential 

 of structure with the typical case here described ; and 

 although there are differences in the details of arrange- 

 ment in the narrower and more rigid leaves of the Conifers 

 Pines, Firs, Yew, Larch, Cedars, Juniper, &c. the same 

 principles are recognisable in these cases also. 



Moreover, the essential features are also similar re- 

 garding the points now to be described. 



Each mesophyll-cell is a delicate vesicle separated 

 from its neighbours, or from any other cell or vessel on 

 which it abuts, by an extremely thin cell-wall or mem- 

 brane, easily permeable by water. Similarly where the 

 cell-membrane abuts on any of the intercellular spaces, 

 its thinness and permeability allow of ready passage of 

 water from cell to intercellular space, or from intercellular 

 space to cell. But the intercellular spaces rarely contain 

 liquid water, though they may occasionally do so. In the 

 normal state of affairs we find them filled with gases and 

 water- vapour only, and the results of thousands of observa- 

 tions and experiments show that the gases are almost 



