272 TISSUE ELEMENTS OF SEED PLANTS 



Cells rich in Protoplasm. Secretory Cells : Protein 

 Cells. Some cells remain densely filled with protoplasm 

 even when adult, or at least contain much more 

 cytoplasm in proportion to cell sap than the average 

 tissue cell. The nucleus is large and conspicuous. 

 Prominent among these are the cells (gland cells or 

 secretory cells) whose special function is the secretion 

 of a definite substance, such for instance as the cells 

 of the nectaries of flowers which secrete a sugary fluid 

 nectar ; as also the cells which produce a particular 

 enzyme, such as are found in seeds where large quantities 

 of stored food material are brought into a soluble 

 form in a short time, 1 and diastase, cytase, or protease 

 is produced in comparatively large quantities. The 

 glands of insectivorous plants, also, secrete proteolytic 

 enzymes which bring the proteins of their victims' 

 bodies into soluble forms. Secretory cells, when active, 

 have large conspicuous nuclei, and are often destitute 

 of a vacuole. They closely resemble in structure and 

 appearance the cells of the glandular epithelium of 

 animals (Fig. 5, B) whose function is the same. 



A kind of cell which resembles these in appearance 

 and structure, and may indeed be said to belong to 



1 It is to be understood that the production of such substances 

 is by no means the monopoly of " secretory cells." They may be 

 produced and are produced in the most various living cells. The cells 

 called secretory are those which are specialised in this direction, and 

 continuously produce large quantities of the substances in question. 



FIG. 43. Living tissue elements of the Seed Plant. A, secreting 

 (glandular) hair arisen from epidermis of leaf. The four cells 

 of the hair are richly protoplasmic and have conspicuous nuclei. 

 Compare with living cells of epidermis and mesophyll below 

 which contain much less protoplasm ; s.h., secretory hair ; n, 

 nucleus; v, vacuole; ep., epidermis; mes, mesophyll; cA.,chloroplast. 

 B, secreting cells from seed of Rye (Secale), with dense granular 

 cytoplasm and conspicuous nucleus (n). C, part of sieve tube 

 in longitudinal section showing two sieve plates (covered with 

 callose) and funnel-shaped slimy cell contents (contracted away 

 from wall). On the right two companion cells (protein cells) 

 with dense cytoplasm and conspicuous nuclei. On the left an 



