ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 387 



these external, there are also internal causes, which he classes under 

 two heads : — " rectipetal," those which tend to straighten the organ in 

 question ; and " curvipetal," those which tend to cause it to curve. 



Distribution of Water in the Heliotropic Parts of Plants.*— 

 A. Thate derives the following conclusions from experiments on 

 Coleus, Clematis, Phaseolus, DaJilia, Sambucus, and Silphium : — 



1. In the parts of plants with positively heliotropic curvature, no 

 difference can be detected between the amount of water in the 

 illuminated and the shaded side. 



2. It cannot, however, be positively asserted that no such difference 

 exists. 



3. Very nearly exact determinations of the amount of water in the 

 parts of plants which curve heliotropically can be obtained by Kraus's 

 method. 



Excretion of Water from Leaves.t — G. Volkens describes the 

 structure of the portion of the tip of the leaf of Calla adapted for the 

 excretion of water. In addition to the ordinary stomata, the epidermis 

 of the cylindrical apex of the leaf is provided with very large modified 

 stomata, which he terms water-fissures. The internal tissue is com- 

 posed of assimilating parenchyma surrounding the extremities of the 

 spiral bundles, which are in close contact with a tissue composed 

 of thin-walled cells with watery contents which the author terms 

 " epithem." The cells of the epithem form a spongy tissue, the large 

 and numerous intercellular spaces of which are always filled with 

 water. The closed ends of the spiral vessels are inserted between the 

 cells of the epithem. The excretion of water in the fluid state on the 

 surface of the leaves is due chiefly to root-pressure. 



The AroideBB is the only order of monocotyledons that possess a 

 true secretory apparatus. In other orders the epidermis gives way at 

 the apex of the leaf, and the vessels discharge their superfluous water 

 into the fissure. Where a special excreting organ is present, it usually 

 occupies the apex and teeth of the leaf; but sometimes, as in Crassula 

 and Urtica, is dispersed over the surface of the leaf. 



Movements of Water in Plants.^ — J- Vesquehas devised a simple 

 method of demonstrating the transfer of water in the stems of plants, 

 which promises to have a wide application. The stem is cut obliquely 

 during immersion in water, and the thin part of the severed stem is 

 placed in the field of the Microscope, of course completely wet on the 

 cut surface. After the cover-glass is adjusted and the stem securely 

 fastened, so that it cannot be easily disturbed by subsequent treatment, 

 a very little freshly precipitated calcium oxalate, or other finely 

 divided substance, is introduced under the cover. If the leaves have 

 not been removed from the stem, a rapid current is at once observed 



* Pringsheim's Jahrb. f. Wiss. Bot., xiii. (1882) pp. 718-29. 



t Volkens, G., ' Ueber Wasserausscheidun^ in liquider Form an den Bliittem 

 hoherer Pflanzen,' 46 pp. (3 pis.) Berlin, 1882. See Bot. Centralbl. xii. (1882) 

 p. 393. 



X Ann. Sci. Nat. (Bot.) xv. (1882) pp. 5-15. See G. L. G. in Amer. Journ. of 

 Sci., XXV. (1883) pp. 237-8. 



2 C 2 



