148 



VEGETABLE FABRIC. 



[lesson 24. 



furnish the invaluable fibres of flax and hemp ; the wood of the 

 stem being tender, brittle, and destroyed by the processes which 

 separate for use the tough and slender bast-cells. 



413. Ducts (Fig. 348-350) are larger than wood-cells, some of 

 them having a calibre large enough to be seen by the naked eye, 



when cut across (407), although 

 they are usually much too small 

 for this. They are either long 

 single cells, or are formed of a row 

 of cells placed end to end. Fig. 

 349, a piece of a large dotted duct, 

 and two of the ducts in Fig. 350, 

 show this by their joints, which 

 mark the boundaries of the several 

 cells they are composed of. 



414. The walls of ducts under the microscope display various 

 kinds of markings. In what are called 



Dotted Ducts (Fig. 348, 349), which are the commonest and the 

 largest of all, — their cut ends making the visible porosity of Oak- 

 ■jrood, — the whole wall is apparently riddled with holes ; but until 

 they become old, these are only thin places. 



Spiral Ducts, or Spiral Vessels, also the varieties of these called 

 Annular or Banded Ducts (Fig. 350), are marked by a delicate fibre 

 spirally coiled, or by rings or bands, thickening the wall. In the 

 genuine spiral duct, the thread may be uncoiled, tearing the trans- 

 parent wall in pieces ; — as" may be seen by breaking most young 

 shoots, or the leaves of Strawberry or Amaryllis, and pulling the 

 broken ends gently asunder, uncoiling these gossamer threads in 

 abundance. In Fig. 355, some of these various sorts of ducts or 

 vessels are shown in their place in the wood. 



415. Milk- Vessels, Turpentine- Vessels, Oil-Receptacles, and the 

 like, are generally canals or cavities formed between or among the 

 cells, and filled with the particular products of the plant. 



FIG. 348. Part of a dotted duct from a Grape-vine. 349. A similar one, evidently com- 

 posed of a row of cells. 350. Part of a bundle of spiral and annular ducts from the stem 

 of Polygonum orieiitale, or Princes' Feather. All highly magr.ified. 



