44 Mr W. Gardiner, On the Physiological [Nov. 12, 



viously be described as excreta, for so far as we know these ele- 

 ments are to be regarded as waste products, and are in all cases 

 got rid of by the plant, and excluded from the general cell-meta- 

 bolism, either by being excreted outside, by means of certain 

 epidermal cells, such as those of Cheiranthus, Limoniastrum or 

 Polypodium, or finally are thrown down in certain cells set apart 

 for the purpose, as in the cells of cystoliths 1 , or the crystal con- 

 taining cells of so many vegetable tissues. 



The glands of such plants as Saxifraga crustata, in their func- 

 tion of excretion of salts of calcium and magnesium, may well 

 be compared to the animal kidney, and it is a matter of interest 

 that we have in the plant as in the animal, distinct excretory 

 glands. With regard to other epithem tissues which appear not 

 to possess such excretory function, it is a question whether the 

 full dignity of glands should be allowed them. For a structure 

 to be truly glandular there must be a distinct secretion or ex- 

 cretion of some kind. In those cases where the epithem tissue 

 merely plays the part of a resistance, or .where, as in Mono- 

 cotyledons it is scarcely developed at all, the water mechani- 

 cally escapes, merely as a result of root-pressure. At present, 

 however, it is best not to urge this point, because we have 

 but few analyses of such exuded water, and we do not know 

 whether pure water is exuded in every instance. On the whole, 

 when we compare the glands of Dicotyledons with those of their 

 Monocotyledonous neighbours, where well-developed gland cells are 

 wanting, it would appear that the chief function of the epithem 

 tissue is to serve as a resistance, and that in certain instances 

 it may act as an excreting tissue 2 . We have no grounds for 

 supposing that the general cell metabolism of Monocotyledons 

 materially differs from that of a Dicotyledon, in virtue of which 

 the latter should require excretory structures which the other did 

 not possess. As regards the activity of the cells of epithems which 

 do excrete salts, it may be remarked that although their tissues 

 are rich in those salts in consequence of a definite excretory 

 capacity, yet they cannot of their own activity get rid of such 

 salts in solution, but depend upon root-pressure and other favour- 

 able conditions for the supply of a force which they cannot of 

 themselves exert. 



1 It would be of interest to observe, whether in the case of cystolith cells there 

 is any external exudation of water, as in Limoniastrum, only that in this case the 

 salts are thrown down in the cell and do not pass through the cell walls. 



2 In those cases where an excretion of chalk takes place, crystals are not 

 present in the tissue of the plant ; where no excretion takes place, e.g. Fuchsia and 

 Urtica, raphides or cystoliths are numerous. In Ficus diversifolia I did not 

 observe cystoliths. The water-glands do not excrete salts, but the small gland 

 on tbe under side of the leaf is covered by scales of what appear to be calcic 

 carbonate. It is small in amount, however. 



