278 MORPHOLOGY. 



but watery juices, often lie between the upper layer and the epi- 

 dermis : more rarely, in like manner, at the under surface of the leaf. 

 Besides these, there appear at given places, or dispersed in the 

 parenchyma, according to special peculiarities of the plant, spiral 

 fibrous cells, very thick, and closely porous cells, and cells contain- 

 ing peculiar juices and crystals. We find also milk-vessels and 

 passages, receptacles for gum, oil, and resin, also isolated liber- 

 bundles, the last especially in the thin elongated leaves of Mono- 

 cotyledons. Air-canals and air cavities are also found in the 

 leaves ; the last very regularly and beautifully arranged. 



Here it is almost as difficult as in the axis, to make any general state- 

 ments. Almost all combinations of forms of the elementary organs, and 

 of the several tissues, are presented in the leaves ; and much confusion 

 has arisen from the attempts which have been made to seize arbitrarily 

 upon some conditions, which, though frequently exhibited, are not in- 

 variable, and to assume these as the type from which till deviations are 

 to be regarded as exceptions. 



Let but the leaves of the Orchidacea be subjected to a complete and 

 close investigation, and such a multiplicity of combinations will be dis- 

 covered, that the attempt to account for them all by laws will be quickly 

 laid aside. The Aloinece, Crassulacece, Ficoidece, Piperacece, Proteacecc, 

 and others, afford similar examples. In many plants we certainly find 

 that division into a parenchyma more elongated, dense and green ; and 

 one expanded in all directions, looser and paler, strongly marked ; but 

 there are innumerable plants in which this is not the case in the 

 Dicotyledons, but particularly in the Monocotyledons : hence it is al- 

 together wrong to assume it to be the regular structure of the leaf. This 

 too could only be done by assuming, in an equally arbitrary manner, that 

 the flat leaf is the regular form. Amongst specialities, which cannot be 

 brought under the general laws, may be enumerated the following : the 

 frequent occurrence of spiral fibrous cells in the leaves of the Orchidacecc 

 of the tropics, and in Gesnera latifolia ;* the same in the stipules of 

 the Paronychiacece ; the peculiar stellate hairs which are projected into the 

 air canals of Nymphcea, Nuphar, Euryale, &c.|, the similar very sin- 

 gular layer of clavate, sometimes ramified, and greatly thickened cells, 

 traversing the layer of elongated parenchyma in the species of Nymphcea, 

 Nuphar, and Hakea ; the thicker or thinner layer of almost colourless 

 cellular tissue, which covers the layer of elongated cellular tissue in 

 many species of Peperomia^ and some of Ficus, whilst plants nearly re- 

 lated to them present nothing of the kind ; the monstrous crystals often 

 extending almost through the entire thickness of the leaf in the Agaves 

 and in Pontederia crassipes ; the cells often projecting into the air-canals 

 on both sides of the septa containing bundles of crystals (Turpin's bifo- 

 rines) in Aroidece., single large crystals in Pontederacea, or groups of crys- 

 tals in Myriophyllum and Proserpinaca ; the air-canals often arranged 

 with such elegant regularity in water and bog plants ; and the air cavities 

 in the leaves of the Grasses J, &c. 



* But in none of its allies which I have been able to investigate. Here \ve may 

 most easily trace the gradual conversion of true spirals into porous structures Avith slit- 

 like pores. 



f In a similar strange manner, also, in a rhizome of Rumex crispus (?). 



\ Even in the very young leaves of the group we find very delicate transparent 



