96 



PART II. ANATOMY AND HISTOLOGY. 



28 



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 G-ymnosperms and Pteridopbyta^.. 

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

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

 associated 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 Solanacese, Campanulacese, and Compositse, and in the 

 cortex of the stem of Cucurbitacese, and some other plants), and 

 generally in roots. Tissue of this kind has been found to be pre- 

 sent in plants so low in the scale as some of the larger Algae 

 (Laminariacese). 



7. Glandular Tissue. Under this general term are included 



cells which 

 produce more 

 or less peculiar 

 subs t ances 

 termed secreta, 

 by a process 

 known as se- 

 cretion. The 

 cells may be 

 isolate^ or 

 they may be 

 collected into 

 groups;; tile 

 secretum may 

 be a c c u m u- 

 lated in the 

 cavity of the 

 secreting cell, 

 or it may be 

 thrown out at 



the surface (excreted) ; the process of secretion may or may not 

 involve the destruction of the secreting cell. 

 The following are the chief varieties of glandular tissue : 

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

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

 :md the nectaries present in ilowers (floral nectaries) or in other 

 parts (extra-floral nectaries) of various plants. In both these 

 forms of gland the secretum, chalk in the one case and sugar in 



C. 



Fio. 76. Sieve-tissue of woody plants. Portions of sieve-tubes 

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

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

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

 si mucilaginous substance ; st starch-granules. B Part of a trans- 

 verse wall seen from the surface. C The same in section ( x 700) : 

 P pits ; c callus; pi the four sieve-plates. 



