ABSORPTION OF FOOD, AND EXCRETION. 499 



had been obtained from without ; but my observations have been few. 

 How far the observations made upon other plants, and mentioned in their 

 morphology, are accurate, I cannot decide. Respecting the anatomical 

 formation of these parts, and the means by which they separate their 

 water, we know much too little. 



I have already remarked, that I can connect no precise and definite idea 

 with the term gland, as referred to a plant. No attentive observer can 

 avoid seeing how different is life in different cells, whether they are 

 found in different plants, or in the same plant, or near each other. It 

 appears to me quite foolish to denominate that cell, or that group of cells, 

 which contains different matter from its neighbours, a gland or organ for 

 secretion ; for there are many plants, and parts of plants, which would 

 then consist only of glands. It is ridiculous to call a cell containing 

 volatile oil a gland, and to refuse the name to one that contains red or 

 yellow colouring matter ; and should we call the last glands, then almost 

 all petals would consist only of glands. The epidermis would be some- 

 times an epidermis but sometimes a glandular surface ; and with many 

 single cells we should have to admit that they are partially glands and 

 partially not so. If we are to retain the term gland at all, we must apply 

 it to those cells and those masses of cellular tissue which, in consequence 

 of some particular structure, not only contain certain fluids, but also secrete 

 them. 



The expression gland, then, must be applied not only to the receptacle 

 of secretions mentioned in the next paragraph, but also to definite groups 

 of cells on the surface of plants, which, not covered by epidermis, and 

 consisting of tender cell- walls, allow their contents to exude externally ; 

 of such are the glands secreting water in the pitchers of Nepenthes, the 

 surfaces secreting chalk in the pits of the leaves of Saxifraga a'izoon, 

 S. longifolia, &c., and almost all the actually secreting nectaries and 

 appendages of the epidermis. 



The last point mentioned in the paragraph has been referred to in other 

 parts of the work, but as yet little is known with regard to it ; we have 

 as yet little pursued the fact of the development of volatile oils (scents) 

 in the blossoms and other parts of plants. Some light is thrown on the 

 subject by Morren, "Rapport sur le Mem. de Mr. Aug. Trinchinetti de 

 odoribus florum, &c.," 1839. (Extrait du torn, vi., No. 5 des Bullet, de 

 TAcademie Royale de Bruxelles). 



198. The second process to be treated of is exhalation. From 

 those parts of plants which are exposed to an atmosphere which is 

 not already perfectly saturated with moisture, a continual evapo- 

 ration of water goes on. The process is purely physical, and, 

 according to accurate investigations, it appears to proceed uninter- 

 ruptedly, according to the dryness and motion of the atmosphere, 

 with the temperature, and the amount of surface exposed to 

 evaporation. It is highly probable that the epidermis permits of 

 no passage to the evaporating water, but that, the evaporation 

 occurring in the neighbouring intercellular spaces, it escapes through 

 the stomates when they are not closed by too strong evaporation 

 and consequent relaxation. From this circumstance the exhaled 

 water is never quite pure, but it contains always a small admixture 

 of vegetable substances which cannot be accurately analysed. 



Besides this evaporation of water, we sometimes find in a very 



