150 Practical Plant Biology. 



dividing partition between two tracheids is extremely thin. Among 

 these scalariform tracheids there are one or more strands of 

 tracheids having thin walls supported by an internal spiral thicken- 

 ing. These tubes are narrower than the scalariform tracheids and 

 being formed first are called protoxylem. The cavities of the 

 tracheids contains only water or dilute solutions. There is no 

 protoplasm or granular contents. 



FIG. w.Aspidium filix-mas, small conducting tract or vascular bundle, 

 transverse section, x 250. c, inner walls of cortex forming bundle 

 sheath ; d, endodermis ; /, pericycle ; p, wood-parenchyma ; s, sieve- 

 tubes in bast ; t, tracheae in wood. 



The phloem, which is the soft layer covering the xylem strand, 

 is chiefly composed of sieve-tubes. Like trie tracheids they too 

 are spindle shaped and polygonal in cross section. Their walls are 

 very thin and are formed of cellulose, and where two sieve-tubes 

 adjoin are marked out into irregular areas the sieve-plates, 

 which are perforated by exceedingly fine pores. The tubes are 

 lined with a very thin film of protoplasm which is continuous with 



