ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 241 



substances is much less than in the leaves. Generally speaking, the 

 leaves of evergreen trees one year old contain a greater quantity of 

 ash than those of herbaceous plants. 



C3) Structure of Tissues. 



Secretion-reservoirs.* — M. F. Jadin has examined the location 

 of the reservoirs of secretions in plants belonging to a large number of 

 different families. The AroidesB have canals, pockets, and cells, all 

 of which play the part of secreting organs. The arrangement in Dico- 

 tyledons may be grouped under the following heads, viz. : — (1) 

 Cortical canals in the root and the stem (some Clusiacese) ; (2) endo- 

 dermal canals in the root and the stem (Compositaa) ; (3) pericyclic 

 canals in the root and the stem (UmbellifersB, Araliacese, Pittosporese, 

 Hypericaceas) ; (4) liber-canals in the root and the stem (Terebin- 

 thacese); (5) liber-canals in the root only (Liquidambaraceee) ; (6) 

 ligneous canals in the root and the stem (Dipterocarpese) ; (7) ligneous 

 canals in the stem only (some Simarubese and Liquidambaracese); (8) 

 medullary canals in the stem only (Bixaceae). The part of the plant 

 in which secreting organs are least often found is the root. 



Reservoirs of Gum in Rhamnacese.t — MM. L. Guignard and Colin 

 have observed in certain Ehamnaceae reservoirs of gum or mucilage, 

 analogous to those found in Malvaceae and Tiliaceae. They are to be 

 met with in Bhamnus, Hovenia, Ceanothus, Palinurus, Zizyphus, Gouania, 

 &c., while they have not been observed in Berchemia, Sarcomphalus, 

 Alphitonia, Colubrina, &c. In every case the reservoirs, whatever their 

 size, can be easily studied with the aid of alcoholic hsematoxylin, which 

 colours the contents. The reservoirs are to be met with either in the 

 stem, leaf, or petiole, or in the pericarp of the fruit ; they, however, 

 appear to be absent from the primary and secondary roots. 



Palisade-parenchyme.l — Herr O. Eberdt has investigated the struc- 

 ture and origin of the palisade-parenchyme in the leaves of a number of 

 species of plants. He dissents from the view of Stahl that this particular 

 form of cell can be called into existence directly by the action of light, 

 regarding it, on the contrary, as in general a hereditary property. Most 

 plants, or especially their leaves, display from the first a disposition to 

 form at least one layer of palisade-cells without the influence of any 

 external agency. This is shown by the existence of this one layer even 

 in leaves found in the deepest shade or in the dark. The lengthening 

 of the palisade-cells and the increase in the number of layers are 

 brought about by the concurrent action of assimilation and transpiration, 

 the length of the cells or the number of layers being in proportion to 

 the extent to which these two forces co-operate. If the amount of 

 transpiration be very small, then, notwithstanding active assimilation, a 

 dissolution of the palisade-parenchyme may take place by the formation 

 of intercellular spaces, and the consequent loosening of the tissue. 



* ' Les organes secreteurs des vegetaux et la matiere me'dicale,' 83 pp. and 3 pis., 

 Montpellier, 1888. See Bull. Bot. Soc. France, xxxvi. (1888) Kev. Bibl., p. 178. 



t Bull. Soc. Bot. France, xxxv. (1888) pp. 325-7. 



X Ber. Deutsch. Bot. Resell., vi. (1888) pp. 360-74. Cf. this Journal, ante, 

 p. 82. 



