CELLS AND TISSUES 35 



which may remain active or also become dissolved. This type is 

 referred to as lysigenous. Lysigenous glands are conspicuous in the 

 leaves and other structures of Gossypium, and form the oil sacs of 

 the citrus fruits. 



Digestive glands occur in many plants as highly specialized 

 structures or as layers of secretory tissue, especially in fruits and 

 seeds which secrete enzymes that digest reserve foods. This is 

 the case in the grain of Zea, where the cotyledon (scutellum) of 

 the embryo is bounded by a layer of digestive cells which forms 

 epithelial glands; and comparable digestive regions occur in the 

 akene of Lactuca. 



The Nectaries. — In plants depending upon insects for pollina- 

 tion, nectaries are commonly present. The secretion of nectar is a 

 function of somewhat papillate epidermal cells which vary in their 

 degree of organization and specialization. These cells are not al- 

 ways limited to specific regions constituting a nectary; and may be 

 unspecialized, in which case the secreting cells resemble other 

 epidermal cells except that they have no cuticle. Although necta- 

 ries are ordinarily associated with floral structures, extra-floral 

 nectaries may occur as in Gossypium, where they develop on the 

 midrib of the leaf and at the bases of the involucral bracts. (See 

 Fig. X14.) 



Lactiferous Cells and Ducts. — A special type of secretory 

 tissue, characterized by the presence of latex, occurs in several 

 plant families. This substance is a complex emulsion that varies 

 greatly with respect to its chemical constituents. Among other 

 substances, it may contain sugars, oils, proteins, tannins, alka- 

 loids, and gums; and, in addition, starch grains and salts may be 

 present. The viscous fluid is usually milky white (Lactuca and 

 Ipomoea); but it may be golden brown (Cannabis), or less fre- 

 quently yellow (Argemone). 



In the Euphorbiaceae, Asclepidaceae, and Apocynaceae, latex 

 may occur in cells which develop in the embryo and finally form a 

 much branched system of tubes. These are morphologically 

 equivalent to a single cell initial which keeps pace with the growth 

 of the apical tissue of the plant. A different type of lactiferous 

 cell is found in the Liliaceae; and, in Allium, these appear early 

 in ontogeny in the cortical region of the leaf. They are arranged 

 in longitudinal rows parallel to the epidermis and are intercon- 

 nected by pits. (See Fig. 91.) A unique form occurs in some spe- 



