TANNIN CELLS 205 



products of the tube itself destined for the useful purpose of 

 healing wounds and giving immunity from parasitic attacks, 

 while others are clearly foods which find in the tubes an efficient 

 means of distribution. 



Tannin Cells. — Tannin cells are found in various families 

 of plants. They occur as single isolated cells or in small groups. 

 The cells are approximately isodiametric or in various degrees 

 of elongation. The longest known occur in the genus Sambucus, 

 where they become twenty or more millimeters in length and 

 sometimes extend through an entire internode. 



Tannin cells are found in the epidermis, primary cortex, peri- 

 cycle, phloem, medullary rays, and in the mesophyll of leaves. 

 They occur in greatest abundance in the cortex and in the tissues 

 of galls. Tannins seem to be by-products set aside in the tannin 

 cells from the general circulation. It is uncertain whether the 

 tannins are ever used to an appreciable extent in nutrition. 

 They seem to be of service, however, in warding off parasites 

 by their aseptic qualities and astringent taste. 



Special Enzyme-Secreting Cells. — In the Crucifcr^, Cap- 

 paridaceae, and a few other families are found special cells de- 

 voted to the secretion of enzymes, such as myrosin. The pun- 

 gency of these plants is due to allylic mustard oil, produced, it is 

 said, at the moment of injury to the plant by the action of my- 

 rosin on the glucoside potassium myronate which is associated 

 with the ferment. Glucose and potassium sulphate are other 

 products of this reaction. 



The digestive glands of insectivorous plants are unique in 

 that their secretions digest animal tissues and are stimulated 

 to activity by the presence of the captive. On the upper side 

 of the leaves of Pinguicula are two kinds of glandular hairs, 

 a long-stalked form secreting a sticky mucilage which holds 

 fast the prey, and a short form, hardly appearing above the epi- 

 dermis, which, when an insect is captured, secretes and pours 

 forth a digestive enzyme (Fig. 115). The short glandular hairs 

 on the leaves of Dionaea muscipula behave like those of 

 Pinguicula. 



