OF THE ELEMEXTARY STUrCTURES. 741 



which the juice flows." Further investigation is therefore neces- 

 sary before we can come to any positive conclusion upon this 

 subject. 



Recently, Trecul has propounded a new theory as to the func- 

 tions of the laticiferous vessels. As already stated (p. 45), Tre- 

 cul believes that he has seen the laticiferous vessels in many 

 milky plants, communicating freel}' with tiie other vessels, and 

 he concludes that they act as venous reservoirs to the circulating 

 fluid, 



4. Functions of Epidermal Tissue. — The special func- 

 tions 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 exhalation of watery 

 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 read}' 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 epiderm's 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 epider- 

 mal cells especially thickened, or protected by a dense layer of 

 cuticle ; whilst in aerial plants growing in very dry or hot re- 

 gions, as the Oleander {fig. Ill), we have fr'^quently an epider- 

 mis of two, three, or more layers of thick-sided cells, and other 

 special contrivances to prevent a too ready exhalation of fluid. 

 The same plants are best fitted for growth in houses, where the air 

 is usually xery dry. While the epidermis may thus be shown to 

 have for its object the restraining of a too abundant exhalation, 

 the stomata are especially 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, catcris 

 jparilms, so will be the exhalation from them. The exact manner 

 in which the stomata act is not readily explained, but it may be 

 always noticed, that when plants are freely supplied with mois- 

 ture, the stomata have their hoxdiev'mg guard-cells distended with 

 fluid, elongated, and curved, so that the orifices between them are 

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

 the bordering cells contract, sti'aighten on their inner surfaces, 

 and thus close the orifices. Under the former condition of sto- 

 mata. there is a ready communication with the external air 

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



