zones. Here the fibrous tissue underlies the chloren- 

 chyma zones, thus forming - a continuous layer of me- 

 chanical tissue around the whole stem, having an aver- 

 age depth of about 0.5mm (50 microns) (figures 3, 

 age depth of about 0.5 mm. (500 microns) (figure 3, 



Embedded in this layer is a tangential series of vas- 

 cular bundles, one underlying each surface ridge and 

 therefore between adjacent flutings. This is the zone 

 of fibres, and it is the material which is chiefly im- 

 portant in paper making, although the other elements 

 occur more or less according to the grade of paper. 

 The Pith (Fundamental Tissue). 



We use this term to embrace all the remaining tissue 

 lying within the zone of fibres, excluding the vascular 

 bundles. The constituent cells are parenchyma about 

 two to four times as long as broad (pa., figure 5), and 

 relatively thin walled. The walls are for the most part 

 however, quite rigid in the dry straw, and they con- 

 tribute no small share to its mechanical qualities in 

 this condition. Only at the innermost limits of the pith 

 they are soft Availed and lax. The condition is due to 

 the dying off of the innermost pith cells as the cavity 

 of the straw is formed during the development of the 

 plant. The condition is well illustrated by the oat 

 (figure 17). 



Fibro- vascular Bundles. 



Lying in the pith is a series of vascular bundles of 

 large size. 6 These bundles are of rather complex struc- 

 ture (figure 24), and have for their function the trans- 

 portation of water and food materials up and down the 

 stem and leaf of the plant. The portion which has to 



6 There is strictly speaking more than one series, but 

 for the purpose of this description it is unnecessary to 

 distinguish between them. 



do chiefly with the movements of water is the xylem 

 or wood (vessels) and can be recognized in the accom- 

 panying figure as four large openings disposed like 

 the eyes, nose and mouth of the face, and surrounded 

 by cells of much smaller caliber. The small thick 

 walled cells between the eyes are also wood vessels. 

 Most of the wood vessels can be recognized in pulp or 

 macerated material by the numerous slit-like pits in the 

 walls. 



The wood-cells are primary and secondary — those 

 first formed during the development of the stem, and 

 those later formed.' The first formed have very thin 

 walls strengthened by rings or spirals (figure 5, x). 

 With the elongation of the stem during growth the thin 

 walls are ruptured, and the rings are in consequence 

 spread apart longitudinally, as are also the turns of the 

 spirals. In pulp, rings and spirals may both be seen. 

 The secondary wood cells are thick walled, but pene- 

 trated by pits which are more or less oval or slit-like 

 in form and regularly disposed in longitudinal series, 

 suggesting the rungs of a ladder (figure 5). The ends 

 of the wood cells are transverse or more or less oblique 

 — at an angle of scarcely more than 45 degrees and 

 are usually open, so that a series of cells make a con- 

 tinuous tube. Between the wood-cells with pitted walls 

 and those which have only ring thickenings, there are 

 various intermediate stages. On the two sides of the 

 primary wood there are some very thin walled elon- 

 gated wood cells with transverse or slightly oblique 

 Avails. These elements are of very delicate structure, 

 and disappear readily under chemical treatment in mak- 

 ing pulp. 



Wood cells form but a very insignificant part of 

 pulp, due to their originally small proportion and to the 

 breaking up of the primary cells into the minute frag- 



H^HHHHHHB 



W^Tf 



Figures 16-19. 



Sectors of transverse section of the terminal internode of barley, oat, rye and wheat, resp. x 14(5. 

 Ch., chlorophyll tissue space, the tissue disintegrated in these sections. 



