298 R. BRAITIIWAITE ON THE HISTOLOGY OF PLANTS. 



process and to empty milk vessels. The receptacles of peculiar 

 secretion are most frequent in the bast part ; the resin canals in 

 the wood of Finns, Abies, or Larix always consist at first of a 

 string of thin-walled unlignified wood parenchyma, enclosing a 

 small intercellular passage, formed by the retiring apart of a few 

 central cells, whose contents in the resting period are starch, which 

 later becomes changed into volatile oil and balsam. In Pinus 

 the thin-wa1led parenchyma surrounding the central intercellular 

 passage, which increases by division, often very early and before 

 its cellulose case becomes lignified, is partly or wholly resorbed in 

 consequence of resin formation. 



The wide milk passages occurring in the bast part of the Vas- 

 cular bundle of Bhus, &c., arise also from strings of thin-walled 

 elongated parenchyma, grouped round an intercellular passage, and 

 represent partly the bast vessels. 



The milk passages in the rind of Alisma, and also the oleo- 

 resinous passages in the middle or in the circumference of the bast 

 part of many Umbellifera3, as Angelica sylvestris, proceed on the 

 contrary from the drawing apart of a string of thin-walled 

 elongated parenchym cells, by which intercellular passages result. 

 The wide air or milk passages of Aroidece, undoubtedly arise from 

 the resorption of one or two wide spiral vessels. In all these in- 

 vestigations it is hardly necessary to observe that very thin trans- 

 verse and longitudinal sections are required, and that we frequently 

 must examine them after treatment by various re-agents ; the 

 amplication also for correct determination of minute structural 

 details will often require powers of 300 to 500 diameters. 



Illustrative Figures —Plate XV. 



1. — Transverse and longitudinal sections from the middle of the stem of a moss, 

 Polytrichum commune. X 360. P, the pith with irregularly thickened 

 cells a, and thin-walled cells, 6. Y, elongated thin-walled cells of the Vas- 

 cular bundles. Fc, parenchyma of stem. 



2. — Transverse section of the Vascular bundle of Alisma Plantago. a, air 

 passage, p, thin pareuchym cells, v, vessels. Bf, bast fibres. Bv, bast 

 vessels. Bp, bast parenchyma. Wf, woody fibres. 



3. — Developement of Turpentine canals. A, transverse section at the apex of a 

 shoot of Pinios Picea, in the rind of which the small group of dividing starch- 

 bearing cells, TFp,has separated from the other chlorophy Hose cells j>, to form 

 the future Turpentine canal, Tc. X 1000. B and C, transverse and longitu- 

 dinal sections of a perfect canal. The internal cavity, as well as the thin- 

 walled cells wp, are filled with semi-fluid resin, while the thin-walled com- 

 pressed cortical cells j? still contain a small quantity of starch. X 800. 



