138 



PART II. THE INTIMATE STRUCTURE OF PtANTS. 



[29. 



and the nectaries present in flowers (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 

 the other, is in solution, and is excreted at the surface. In the 

 chalk-gland the secretum escapes through a special channel, a 

 water- stoma (Fig. 100 w\ see also Fig. 121, p. 157). In the nectary 

 the secretum is simply poured out on the surface of the gland. 



Chalk-glands are remarkable in that they are developed from the same primary 

 tissue as the vascular bundles, in connexion with which they always occur. 

 These glands belong in fact to the vascular tissue-system (see p. 184). 



FIG. 99. Sieve-tissue of woody plants. Surface-view of parts of two sieve-tubes of 

 Pinus sylvestris, with sieve-plates on the lateral longitudinal walls. Parts of walls of these 

 sieve-tubes treated with iodised chloride of zinc: A before the formation of a callus-plate; 

 B after the closure of the sieves by callus; C an old sieve-tube which is no longer active, 

 and from which all trace of callus has disappeared, (x 540 : after Strasburger.) 



(6) Hollow multicellular glands are intercellular spaces sur- 

 rounded by secreting cells, and are, in some cases, of schizogenous, 

 in others of lysigenous, origin (see p. 130.) The secretum may be 

 mucilage, or gum, or a mixture of gum and resin (gum-resin), or 

 ethereal oil, or a mixture of ethereal oil and resin (balsam). The 

 cavities are either rounded closed spaces, or are elongated canals, 

 extending for some distance through the tissue ; the former are 

 usually of lysigenous, the latter of schizogenous, origin. 



As examples of lysigenous hollow glands, may be mentioned the 

 cavities filled with gum, which occur in the tissue of Cherry-trees ; 

 the oil-glands of the Orange and Lemon, and in the leaves of the 

 Rutaceas, Myrtaceae, and Hypericum, where they can be discerned 



