494 SECRETORY AND EXCRETORY SYSTEMS 



surrounding mesophyll cells in several particulars ; besides being 

 distinctly smaller, they contain abundant protoplasm and a large 

 nucleus, but little or no chlorophyll. Typically they are more or less 

 isodiametric, and either rounded or polyhedral ; not infrequently they 

 are provided with spherical or irregular extensions (Papaver, Geranium), 

 thus approaching the condition of spongy parenchyma (Figs. 198,199 c). 



Hydathode of a leaf-tooth of Primula sinensis in R.L.S. ; o, adaxial ; u, abaxial 

 epidermis ; p, palisade-ceils ; t, terminal traebeides of a bundle-end, projecting freely 

 into intercellular spaces (?) of the epithem () ; s, guard-cell of the water-pore. 



As was first pointed out by Volkens, every epithem-tissue is 

 traversed by a more or less well-developed system of intercellular 

 spaces. The author himself has demonstrated, in the case of Fuchsia, 

 that the distal extremities of the terminal tracheides may abut directly 

 against these intercellular spaces. As a rule, the spaces in question 

 are filled with water, and open into one or more sub-epidermal 

 chambers ; these chambers in their turn communicate with the exterior 

 through open water-stomata that differ, more or less obviously, from 

 the genuine ventilating stomata from which they are phylogenetically 

 derived. The most important difference between the two structures 

 consists in the fact that the guard-cells of water-stomata have lost 



