CHAPTER XII 



Secretory Organs 



Referenxe has already been made in Chapter V to the various 

 by-products which are formed during the constructive and de- 

 structive processes in plants and which often collect as so-called 

 secretions. This term is also applied to the sugary liquid pro- 

 duced by nectaries (both floral and extrafioral) , the digestive 

 juices formed by insectivorous plants, and even the watery 

 exudations appearing at different points on leaves under certain 

 conditions. These diverse secretions are very commonly the 

 result of the activity of specialised cells or groups of cells, which 

 may be classed under the general heading of secretory organs or 

 glands. Their products may pass direct to the exterior, or may 

 be retained in special cavities or canals within the body of the 

 plant. Many of the secretory organs are superficial, and some 

 of these are developed as hairs, for instance the glandular hairs, 

 already described (p. 105), which secrete various by-products. 



Another widespread type, known as a hydathode, serves to 

 remove excess water, either as liquid or vapour. In some cases 

 these hydathodes take the form of glandular hairs, their 

 cells possessing dense protoplasmic contents and large nuclei 

 (Fig. 71, A). Good examples are found on the leaves of the 

 Runner Bean, where they are bent, club-shaped structures situated 

 near the veins and consisting of a row of thin-walled cells, the 

 terminal cell often being divided into two by a vertical wall 

 (Fig. 71, A). Many semi-parasites (e.g. Yellow Rattle, Bartsia, 

 etc.) bear numerous water-secreting hairs situated chiefly along 

 the veins of the leaves. These hydathodes (Fig. 71, B) have 

 the form of small domes consisting of living cells, viz. a large 

 basal cell [b.c.) embedded in the epidermis, a short stalk-cell {s.c), 

 and a head composed of two to four cells lying side b}' side 



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