CHAP, i.] PLANT ARCHITECTURE. 51 



and by the ends being only slightly oblique and not 

 tapering ; they originate from superposed rows of cells, 

 some of the transverse septa being absorbed ; hence the 

 structure called a vessel consists of two or three cells 

 thrown into one by the disappearance of their transverse 

 walls. Vessels formed in the primary wood, or wood of 

 the first year, frequently have their walls thickened 

 internally by the deposition of strengthening material 

 arranged in the form of a loose spiral, and are known 

 as spiral vessels, while vessels that are formed in the 

 secondary wood that is, all wood or xylem formed after 

 the first year, usually, have pitted walls. Ducts are only 

 vessels of extra large diameter, and furnished with 

 pitted walls. Vessels usually contain air, but when the 

 ascent of sap is very rapid, as in the spring, they some- 

 times contain water. 



Phloem or bast, like the xylem, consists of two distinct 

 elements; sieve-tubes, with thin side walls, but having 

 the transverse septa much thickened and perforated with 

 numerous minute holes ; these thickened and perforated 

 septa are called sieve-plates. Sieve- tubes, or soft bast> 

 correspond to the vessels present in the xylem, and 

 contain albuminous substances; bast-fibres, constituting 

 the hard bast, consist of very much elongated, thick- 

 walled cells with tapering extremities, and correspond 

 to the tracheides of the xylem, but differ from the latter 

 in their walls, although usually very much thickened, not 

 becoming rigid, but remaining pliant. The hard bast is 

 the part used in most textile fabrics, as " linen/' "jute/' 

 " hemp/ 3 " Russia matting," etc., " cotton " is an 

 exception to the above, and consists of long, thin- walled 

 cells of cellular tissue that spring from the testa or 



