580 PHYSIOLOGY. 



Secretions. The phenomena of growth have been dwelt upon 

 incidentally in preceding Sections, and are further discussed in the 

 next chapter ; but we have here to speak of certain processes, occur- 

 ring more or less extensively in plants, contemporaneously with 

 growth, in which products are formed which are not, like starch, 

 chlorophyll, &c., evidently transitory forms of assimilated substance. 

 These substances, called by the general name of secretions, are of most 

 varied kinds, and their relation to the economy of vegetable life is 

 very obscure ; but a brief notice of the most striking of them is 

 indispensable. 



A distinction is sometimes made between the peculiar products found 

 in the interior of cells, and those which are accumulated in certain cases 

 in intercellular passages or cavities, or upon the outer surface of cell- 

 membranes, the former being called secretions and the latter excretions. 



The principal substances secreted by plants are air, water, gum, sugar, 

 volatile oils, balsams, resins, gum-resins, and salts, either entirely inor- 

 ganic, or formed of combinations of mineral bases with organic acids, &c. ; 

 besides these there occur in individual Orders a multitude of alkaloids, 

 neutral substances of various kinds, colouring-principles, &c. 



Gases. The liberation of gases into intercellular passages, cavities, &c. 

 occurs both as a necessary accompaniment to the chemical decompositions 

 going on in the cells, and as a special process connected with peculiar 

 habit of plants &c., as in the Utricularice, in the air-sacs of Fucus vesicu- 

 losuSj &c. The composition of the air found in the cavities of plants 

 necessarily depends upon the external conditions, as under sunlight there 

 is generally a greater proportion of oxygen than exists in common air, in 

 the dark but excess of carbonic dioxide. 



Water. Water is given off in a liquid form by various plants, either 

 from glandulir papilla^ or from the general surface of leaves &c. In 

 Nepenthes distillitoria, Sarracenia, &c. water is secreted in the pitchers 

 wherein it accumulates. The leaves of various Musacea3, Aracese, Grasses 

 and other Monocotyledons, Z'ropeeolum, Impatiens, Brassica oleracea, &c. 

 give off drops of water from the leaves. In Caladium there exist orifices 

 at the points of the leaves, communicating with internal canals, whence 

 great quantities of water flow (half a pint in one night). This water is 

 of course contaminated with salts and small quantities of soluble organic 

 matters. 



Gum is usually poured out into and accumulated in intercellular pas- 

 sages, as in the Cycadacea3, in the bark of the Acacias, Cherry, &c. 

 When it is formed in large quantities, it bursts the tissues and exudes in 

 the form of tears. The formation of the gum Tragacanth in the species 

 of Astragalus is different from this, consisting of collenchymatous thick- 

 ening of the cells of the pith and medullary rays, which swell by absorp- 

 tion of water, and burst out from the stem under certain circumstances. 

 The peculiar organs called cystolithes have a gummy excretion as a basis, 

 in the form of a clavate body, suspended in the interior of an enlarged 

 cell by a cellulose pedicle ; when mature these bodies are covered with 



