ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 645 



Heterogeneous pith is the result when some of the cells still remain 

 active at a later period, performing the function of a reservoir of food- 

 material. The walls of these cells are usually thicker and more 

 pitted : they are mostly the peripheral cells. 



It is not unfrequently the case that while some of the pith-cells 

 lignify, others still show the cellulose-reaction. It is then always 

 the central part of the pith which remains unchanged, while the peri- 

 pheral cells lignify early and often become very thick-walled. In 

 other cases finally the pith-cells do not lignify at all, and then later 

 changes sometimes occur, such as the disappearance of older cells and 

 the appearance of new ones in their place. When special elements 

 are found in the pith, such as laticiferons vessels, resin-passages, 

 tannin-receptacles, calcium oxalate, &c, these almost invariably 

 occur also in the cortex. 



It is very rare for the pith to remain entirely unchanged. The 

 pith of climbing shows no important differences from that of non- 

 climbing plants. 



Mechanical Sheaths of Secreting Vessels.* — Dr. M. Mcobius has 

 'investigated the anatomical details where intercellular secreting vessels 

 are surrounded by thickened cells, in a number of examples, especially 

 a large number of species of Pinus, the roots of Philodendron, the 

 stem of the ivy, the ovary of different species of Bromeliaceas, the 

 leaf-stalk of Angioj)teris, &c. 



The species of Pinus examined may be divided into three classes : 

 — (1) Those in which the resin-passages are surrounded by scleren- 

 chymatous bast-like cells ; (2) surrounded by thin-walled cells with 

 some bast-like cells intermixed ; (3) surrounded by thin-wajled cells 

 only. In all the species of Philodendron examined, with one excep- 

 tion, the epithelium of the secreting vessels in the adventitious roots 

 is surrounded by sclerenchymatous cells, forming a partially open or 

 an entirely closed sheath. Three different modifications of this sheath 

 are described. The author believes the function of the sheath to be 

 in all cases a purely mechanical one. 



Medullary Rays of Dicotyledons.! — Herr E. Zache has made a 

 comparative examination of the medullary rays in thirteen different 

 species of dicotyledonous trees and shrubs. He finds the number in 

 a square mm. to vary, as a general rule, between thirty and sixty, a 

 strong deviation occurring in one direction in Platanus orientalis with 

 eight, and in the other direction in Prunus Padus and Castanea vesca 

 with ninety. 



Normal Root-buds.J— By normal root-buds Dr. M. W. Beyerinck 

 means such as are formed in some plants during normal growth, not 

 as the result of wounds such as those formed in callus. In Populas 

 alba and Geranium sanguineum the author observed root-buds intor- 



* Pringsheim's Jahrb. f. Wiss. Bot., xvi. (1885) pp. 262-301 (1 pi.). 



t Zeitschr. f. Naturwiss., v. (1886) pp. 1-29. Cf. this Journal, v. (1885) 

 p. 826. 



X Nederl. Kruidk. Arch., 1885, p. 162. See Bot. Centralbl., xxv. (1S8G) 

 p. 296. 



