828 SUMMARY OF CURRENT RESEARCHES RELATING TO 



a single circle in the median zone, their number amounting to 20, 40, or 

 even 60 ; those of the pith are also most often in a single circle in the 

 peripheral zone, and are always separated by some rows of medullary 

 cells from the primary xylem of the vascular bundles ; sometimes 

 they form two concentric circles. In some genera the cortical canals 

 are wanting. Those genera which have canals in both the cortex and 

 the pith possess them besides in the external and internal paren- 

 chyma of the leaf-stalk ; but when the cortex of the stem is destitute 

 of these canals, so also is the external parenchyma of the leaf-stalk. 

 The large starchy cotyledons of Cola and Heritieria contain no gum- 

 canals ; and in some genera of Sterculiaceae they are entirely wanting 

 even in the leaves and stem. 



Striated Woody Tissue.* — Herr F. v. Hohnel describes a pecu- 

 liarity of the wood of a certain number of exotic trees belonging 

 principally to the natural orders Leguminosse, BignoniacesB, Sima- 

 rubese, and EbenacesB, a striated appearance on longitudinal section 

 due to the presence of horizontal rays. This results from the regular 

 disposition of the medullary rays, which form so many parallel hori- 

 zontal bands. In addition the tracheids, which constitute the 

 greater part of the wood, are swollen in their middle and taper off at 

 their two extremities ; the median swellings which alone are punctated, 

 are arranged in parallel lines, which further contributes to give the 

 wood its striated appearance. 



Structure of Stem of Strychnos.f — M. J. Herail has investi- 

 gated the peculiar structure of the stem of Strychnos, in the three 

 species S. triplinerve, hrasiliense, and nux-vomica, in which the ring 

 of wood is traversed by light-coloured plates sometimes arranged 

 regularly in a circle, sometimes irregularly in the woody mass, com- 

 posed of sieve-tubes surrounded by parenchyma. M. Herail shows 

 that this structure is not due, as has been supposed, to an abnormal 

 power of the cambium of producing both xylem and phloem on one 

 side ; but that it, in the usual way, produces xylem only on one side 

 and phloem only on the other side. The peculiar appearance results 

 from the cambiiim ceasing to produce xylem at the points where it 

 produces phloem; and this peculiarity persisting for an indefinite 

 time, the result is that the xylem formed in all the other parts 

 advances more and more upon the phloem, which it appears ultimately 

 to surround. 



Mechanical Tisssue-system.1: — Dr. A. Tschirch proposes the 

 following terminology for the various developments of this system. 

 By the term sclereid he understands all thick- walled elements which 

 cannot be included under specific mechanical cells, viz. bast-fibres, 

 libriform, and coUenchyma ; hence all the cells formerly classed as 

 sclerenchyma, except the steroids, collenchyma, and libriform cells. 

 The sclereids must also be regarded as specifically mechanical cells, 



* SB. K. Akad. Wiss. Wien, Ixxxix. (1884) pp. 30-47. See Bull, Soc. Eot. 

 France, xxxii. CI 885) Eev. Bibl., p. 9. 



t Bull. Soc' Bot. Fiance, xxxii. (1885) pp. 92-5. 

 J Ber. Deutsch. Bot. Gesell., iii. (1885) pp. 73-5. 



