THE PRIMARY PERMANENT TISSUES 



37 



The contents of the sieve tubes are found to be rich in pro- 

 teids, amido-acids, and soluble carbohydrates, and minute starch 

 grains may sometimes be present in abundance. Even proteids 

 that are in solution do not pass readily through cell-walls, and in 

 the sieve tubes the perforations 

 allow them to pass in an unob- 

 structed stream from one cell or 

 sieve tube member to the other. 

 That the sieve tubes are for the 

 vertical flow of proteids and allied 

 substances is shown by direct ob- 

 servation under the microscope 

 while using suitable reagents for 

 the demonstration of proteids; and 

 further by girdling and constric- 

 tion experiments described in 

 Chapter X. 



The sieve parenchyma cells in 

 differentiating from the procam- 

 bium elongate vertically more or 

 less and increase in their cross 

 diameters (Fig. i8), but they do 

 not become, as a rule, so large in 

 any dimension as the cells of the 

 sieve tubes. Their walls remain 

 cellulose and commonly thicken 

 but little. They appear to serve 

 chiefly in the translocation of car- 

 bohydrates and as storage places 

 for proteids which they are in position to take frolm the sieve 

 tubes when a surplus is at hand, and they assist in delivering 

 over to the medullary rays materials from the sieve tubes to be 

 stored by the rays or transported inward for storage in the cells 

 of the wood or xylem parenchyma. 



The sieve tubes, companion cells, and sieve parenchyma cells 

 seem to remain alive and functional throughout the first year of 



Fig. i8. — ^Stages in the development 

 of sieve tubes, companion cells, and 

 phloem parenchyma. A, a and b, 

 two rows of procambial cells; in c and 

 d, a has divided longitudinally and c 

 is to become companion cells; d, a 

 sieve tube, and b, phloem paren- 

 chyma; B, c, companion cells, and d, 

 a beginning sieve tube from c and d, 

 respectively in A. The cross-walls 

 in d are pitted; b, phloem parenchyma 

 grown larger than in A; C, the same 

 as B with the pits in the cross-walls of 

 the sieve tubes become perforations, 

 and the nuclei gone from the cells 

 composing the tube. 



