l86 LECTURE XL 



pendiculaily to the surface of the leaf, and first exhibit the splitting and separation 

 of the secretion mentioned in the middle: this proceeds so far later on, that. the 

 individual cells of the complex appear to lie quite separate in the mass of secreted 

 substances (De Bary). 



Johannes Hanstein distinguished as Colleters certain massive multicellular 

 hairs, the secretion of which consists of resin and gum, or gum alone, and 

 effects the sticking together of the parts of the bud in many plants. This secre- 

 tion is very conspicuous on the just opening buds of the Horse-chestnut, Poplar, 

 and Sjringa ; and is evident in a less degree in very many other woody plants 

 {Ribes, Corylus, Carptnus, Lomcera, Sarnbucus), and also in many herbaceous plants 

 {Heliajithus, Datura, Salvia, Viola, &c.). In the Polygoneae, especially the Docks 

 and Rhubarbs, it is chiefly mucilage, which becomes diffluent in water and fills 

 up the spaces between the folded parts of the bud and covers the developing 

 leaves and buds. These secretions, which make their appearance in the very young 

 parts of the bud, are lost on the complete unfolding of the leaves and internodes, and 

 appear to be a means of protection for the young organs against drying up, and 

 other injurious influences. These substances are excreted from the hairs or colleters 

 mentioned, which are developed especially at the edges of the young stipules, 

 and elsewhere on the parts of the bud. They may present the most various 

 forms, but the secretion always takes place between the cuticle and the outer cell-wall 

 of the body of the hair : the cuticle, as in other cases, is raised up in a vesicular 

 manner by the secretion, until it finally bursts (especially on the admission of 

 water) by the swelling of the slime, and the mass spreads over the surface of 

 the organ. Examples of schizogenous glands occur here also; and occasionally 

 the epidermis itself is concerned in the formation of the secretion between its 

 cuticle and cell-v/all. 



In connection with the glandular hairs may be briefly mentioned here the 

 mealy, dust-like, capitate hairs, described by De Bary, to which the under surface 

 of the leaf of the so-called Gold and Silver Ferns {Gymnogramnie, Noihochlaena, 

 Cheilatithes) owe their white or golden yellow {Pieris aurala) meal-like covering. 

 The same is true of the mealy-dusted foliage leaves of many Primroses, e.g. Primula 

 marginata, farinosa, auricula. The mealy covering is exclusively produced by 

 the round capitula of shortly-stalked hairs ; from all points of the surface of these 

 capitula are developed very thin, but relatively long rodlets or threads of an 

 apparently resinous substance, which dissolves in alcohol, ether, acetic acid, and 

 alkalies, and (it is said) again crystallises out from the solutions. This mode of excre- 

 tion appears to be only another form of the excretion of balsam occurring elsewhere 

 between the cuticle and cell-wall of the hair; since, according to De Bary, the 

 vesicular glandular hairs previously described may occur in place of these dusty 

 hairs on the leaves of the same or allied plants. 



Physiologically the most remarkable of all external glands are the digestive 

 glands of the insectivorous plants. I contemplate treating of these later, however, 

 when dealing with the theory of nutrition. 



