514 SECRETORY AND EXCRETORY SYSTEMS 



secretion is derived directly from the cell-contents. As a matter of 

 fact, numerous highly refractive vesicles and globules of various sizes 

 make their appearance in the protoplast both before and during the 

 process of secretion ; according to Tunmann, these bodies consist either 

 of fat or of tannin. But in whatever fashion secretion may take place, 

 the chemical changes involved in the process are undoubtedly controlled 

 by the living protoplasts of the glandular cells. It is impossible, at 

 the present time, to suggest how this influence is transmitted across the 

 [apparently unaltered] innermost layer of the cell-wall. 



In certain cases the secretion seems to originate in the cell-cavity, 

 and to escape directly to the exterior without producing any rupture of 

 the cuticle. According to Behrens, this case is exemplified by the 

 glandular hairs of Ononis spinosa. Here the mobile ethereal oil which 

 constitutes the secretion, first becomes visible in the protoplasm of the 

 secretory cells, and later appears on the outer surface of the " head " in 

 the shape of numerous small drops adhering to the cuticle ; these drops 

 continually increase in number and gradually coalesce to form a single 

 large drop, which ultimately falls off. The glandular hairs of Cicer 

 arietinum and those of certain Onageaceae which secrete an acid liquid, 

 probably belong to the same category ; for Stahl states that fresh drops 

 of the acid substance appear on the heads of these hairs even when the 

 secretion has been repeatedly washed off with water. The " mealy " 

 hairs found in the so-called " Gold " and " Silver Ferns " {Gymnogramme 

 spp.) and in certain species of Primula (P. Auricula, P. farinosa, etc.), 

 are probably of a similar nature. The mealy coating of the shortly- 

 stalked capitate hairs consists of slender, often more or less curved, 

 rods and needles of a resinous or fatty substance ; in Gymnogramme, 

 according to De Bary, these bodies are attached to the outside of the 

 smooth cuticle. 



The physiological and ecological significance of the dermal glands 

 that have been described in the preceding paragraphs, varies according 

 to the nature of the substance secreted. It is possible that in certain 

 cases the secretion merely represents a useless by-product of metabolism, 

 which must be got rid of by some means or other ; apart from this 

 possibility, however, most of the secretions in question probably serve 

 to keep down transpiration, or to ward off the attacks of injurious 

 animals. With regard to the first-mentioned function, Hanstein long 

 ago pointed out that the mucilage hairs or " colleters " which occur on so 

 many bud-scales, and on young foliar organs in general, afford protection 

 against excessive transpiration and other injurious influences by virtue of 

 the secretion (a mixture of gum and ethereal oil or resin) which they pour 

 over the tender developing organs. The adult leaves of many xero- 

 phytes are similarly covered sometimes on both sides, sometimes ov 



