728 PHYSIOLOGY. 



functions of the laticiferous vessels. As already stated (p. 38), 

 Trecul believes that he has seen the laticiferous vessels in many 

 milky plants, communicating freely with the other vessels, and. 

 he concludes that they act as venous reservoirs to the circulating 

 fluid. Trecul having kindly afforded us an opportunity of 

 examining several of his microscopic preparations, which he 

 believed would prove the connection of the true vessels with the 

 laticiferous, we are bound to state, that we could not satisfac- 

 torily trace any such a union as he has described. 



4. Functions of Epidermal Tissue and its Appendages. 

 — The special functions of these parts are : — to protect the 

 tissues beneath from injury, and from being too rapidly affected 

 by atmospheric changes ; to regulate the transpiration or ex- 

 halation of Avatery fluids ; to absorb and exhale gaseous matters ; 

 and probably, to some extent, to absorb water. The epidermis 

 itself, is specially designed to prevent a too ready evaporation 

 of fluid matters from the tissues beneath, and hence, we find, 

 that it is variously modified to suit the different conditions to 

 which plants are submitted. Thus, in submersed plants, which 

 are always exposed to similar influences as regards moisture, 

 there is no true epidermis; whilst in aerial plants submitted to 

 ordinary influences in cold and temperate climates, we generally 

 find an epidermis with only one layer of thin-sided cells, and 

 covered by a cuticle of only moderate thickness. In other aerial 

 plants, however, growing in the same latitudes, such as the Box, 

 &c., and generally in those of a succulent nature where there is but 

 a moderate exhalation, we find the upper walls of the epidermal 

 cells especially thickened, or protected by a dense layer of 

 cuticle ; whilst in aerial plants groAving in very dry regions, 

 and more especially in those of warm and tropical climates, as 

 the Oleander {fig. 81), we have frequently an epidermis of two, 

 three, or more layers of thick- sided cells, and other special con- 

 trivances to prevent a too ready exhalation of fluids. While 

 the epidermis may thus be shown to have for its object the 

 restraining of a too abundant exhalation, the stomata are espe- 

 cially designed to facilitate and regulate the passage of fluid 

 matters, and in proportion to their number, therefore, upon the 

 different organs and parts of plants, cateris paribus, so will be 

 the exhalation from them. The exact manner in which the 

 stomata act is not readily explained, but it may be always no- 

 ticed, that when plants are freely supplied with moisture, that 

 the stomata have their bordering cells distended with fluid, 

 and curved outwards, so that the orifices between them are 

 open ; whilst in those cases, where there is a deficiency of fluid, 

 the ])ordcring cells contract, straighten on their inner surfaces, 

 and thus close the orifices. Under the former condition of 

 stomata, there is a ready communication with the external air 

 and the internal tissues, and hence a free exhalation takes place; 



