ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 383 



from Traube's statement of tlie changes vvMcli accompany oxidation, 

 especially the formation of peroxide of hydrogen, J. Keinke gives 

 the following as a sufficient basis on which to build a theory of 

 oxidation in living cells. (He has himself shown that there exists in 

 certain plants, notably in the beet, a very easily oxidizable body, 

 which he has named rhodogen. This substance is one of Traube's 

 autoxidizable bodies, and is only one of many which may be reason- 

 ably assumed to be present in cells.) 



1. In every active cell, autoxidators are formed ; that is, sub- 

 stances which, at a low temperature and with absorption of molecular 

 oxygen, can be oxidized by the decomposition of water. 



2. By oxidation of these substances, peroxide of hydrogen is 

 produced. 



3. This peroxide of hydrogen can, under the influence of diastase, 

 and probably of other ferments, cause oxidations as energetic as atomic 

 oxygen can. 



Lastly, the seat of this activity is the periphery of the proto- 

 plasmic body of the cell ; and this body possesses an alkaline 

 reaction. 



Structure of the Bundle-sheath.* — The cells of the typical 

 bundle-sheath are, according to S. Schwendener, parenchymatous and 

 of variable length ; the pores, when present, are mostly round, though 

 occasionally oval and oblique. Like the mestome-bundles, the sheaths 

 form a continuous system. The impermeability of the walls of the 

 sheaths is no invariable characteristic, since portions only of them are 

 often cuticularized. It is, however, the rule for the bundle-sheaths 

 to be, when mature, less permeable than ordinary cellular tissue, this 

 being due to a relatively impermeable lamella, which bounds the 

 inner surface of their tangential walls ; in consequence of this the 

 sheaths often assume the functions of the epidermis. The formation 

 of pores on the inside of the sheaths stands in close relationship to 

 the mode in which their permeability decreases in the course of their 

 development. When the inner wall has become so thick as to be 

 impermeable there are no pores. 



In many monocotyledons, dicotyledons, and ferns, the cells of the 

 bundle-sheath of the root are of two kinds ; opposite the primordial 

 vessels are " transmission-spots " more permeable than the rest of the 

 sheath, from the cells having thinner walls. The vessels are water- 

 conducting tubes, and these transmission-spots serve to keep up a 

 connection between this system and the fresh bark ; they are the 

 sluices of a system of irrigation. In the mestome-sheath of mono- 

 cotyledons these passages are found at two symmetrical points, while 

 in ferns they always correspond, in number and position, to the 

 groups of primordial bundles. These passages appear never to occur 

 in the bundle-sheaths of rhizomes. 



Besides the suberization of the radial and transverse walls of the 

 bundle-sheaths, the tangential walls remaining unchanged, Schwen- 



* Abhandl. K. Akad. Wiss. Berlin, 18S2 (5 pis.). See Bot. Ceiitralbl., xiii. 

 (1883) p. 77. 



