29.] 



CHAPTER II. THE TISSUES. 



137 



With the sieve-tubes of Angiosperms are closely associated cells, 

 termed companion-cells (Fig. 97 s), which are filled with granular 

 proteid contents and have well-marked nuclei ; each companion- 

 cell is of common origin with the corresponding segment of a 

 sieve-tube, both being derived from one mother-cell. Companion- 

 cells are not developed in Grymnosperms and Pteridophyta. 



The sieve-tissue, like the tracheal tissue, is a characteristic 

 constituent of the vascular tissue-system : it is very frequently as- 

 sociated with tracheal tissue so as to form one vascular bundle, but 

 it may occur in independent bundles (e.g. in the pith of the stem 

 of some Solanaceae, Campanulaceae, and Compositae, and in the cor- 

 tex of the stem of Cucurbitaceaa, and some other plants), and 

 generally in 

 roots. Ifc serves 

 for the conduc- 

 tion of organic 

 sub s t an ce, 

 more especi- 

 ally of proteid 

 substance. Tis- 

 sue of this kind 

 has been found 

 to be present 

 in plants so 

 low in the 

 scale as some 

 of the larger 

 Algae (Lami- 

 nariacece). 



7. Glandular 

 Tissue. Under 

 this general 



term are included cells which produce more or less peculiar sub- 

 stances termed secreta, by a process known as secretion. The cells 

 may be isolated, or they may be collected into groups ; the secre- 

 tum may be accumulated in the cavity of the secreting cell, or it 

 may be thrown out at the surface (excreted) ; the process of secre- 

 tion may or may not involve the destruction of the secreting cell. 



The following are the chief varieties of glandular tissue : 



(a) Solid multicellular glands. Good examples of these are the 

 chalk-glands of the leaves of many Saxifragacese and Crassulacese, 



c. 



FIG. 98. Sieve-tissue of woody plants. Portions of sieve-tabes 

 from the secondary bast of the Vine. A Entire transverse wall and 

 adjacent parts in longitudinal section (x300): pi the sieve-plates; 

 ft the thicker portions of the cell-wall ; h the protoplasmic lining ; 

 si gelatinous substance ; st starch-granules. B Part of a transverse 

 wall seen from the surface. C The same in section (x 700) ; p pitsj 

 c callus ; pi the four sieve-plates. 



