DERMAL GLANDS 511 



the clavate hairs of the extra-nuptial nectaries have exactly the same 

 structure as the clavate trichome-hydathodes which occur on the upper 

 side of the young pinnae. 



D. OIL-, RESIN-, MUCILAGE- AND GUM-SECRETING GLANDS. 



1. External or dermal glands. 2 



The glandular organs which secrete oil, resin, mucilage, or gum are 

 frequently epidermal in a morphological sense, and are thus comparable 

 to epidermal hydathodes and nectaries. 



In the simplest cases secretion of such compounds is carried on by 

 ordinary unspecialised epidermal cells. The so-called glandular surfaces 

 produced in this way are more especially characteristic of such bud- 

 scales as are, in addition, provided with glandular hairs (e.g. Rumex, 

 Rheum, Coffea, Alnics, Betida, etc.). In certain species of Silene a sticky 

 area is developed below each node. Within these areas specialised 

 cells occur interspersed among the ordinary epidermal elements. In 

 SUene nemoralis, according to Unger, these special cells are broader 

 than the other epidermal cells, and are further distinguished by their 

 papillose outer walls and abundant granular protoplasmic contents ; it 

 is highly probable that they represent the actual secreting elements. 

 The modification of epidermal cells in relation to a secretory function is 

 more far-reaching and conspicuous in the case of the so-called glandular 

 spots, small, sharply -defined secretory areas which occur more parti- 

 cularly on the leaf- teeth of various plants (spp. of Prunus and Salix, 

 Ricinus, etc.). The secretory elements of which such a glandular spot 

 is composed, are usually more or less columnar (prismatic) in form and 

 are generally arranged in palisade-like layers ; they are further 

 distinguished from the neighbouring epidermal cells by their very thin 

 walls and dense protoplasmic contents. Mention may finally be made, 

 at this stage, of the so-called intra-mural glands of the genus Psoralea. 

 Here the more or less spherical " body " of the epidermal gland is 

 made up of a considerable number of tubular secretory elements, some 

 of which are curved in a radial plane, while others are irregularly 

 sinuate. 



Glandular trichomes exhibit an extraordinary diversity of form. 

 A typical glandular hair consists of two distinct portions in addition 

 to the base or foot, namely : first, a one- to several-celled stalk of 

 varying length ; and, secondly, the actual secretory organ or gland, 

 which takes the form of a rounded head attached to the distal end of 

 the stalk. The head may be unicellular, as in Pelargonium zonale 

 (Fig. 207 a) and Primula sinensis; or it may be divided into quadrants, 

 as in Lamiuvt, Plectranthus, etc. ; or, finally, it may be composed of a 



