OF THE ROOT OE DESCEXDIKG AXIS. 743 



of tnie epidermal cells, which are subseqiiently placed on a level 

 ■with these, or become pushed downwards or upwards, according 

 to the ultimate position of the stomata. The frequent occur- 

 rence of chlorophyll in the stomatal cells seems to support 

 Mohl's view. While observers diifer as to the origin of the sto- 

 matal cells, they are all agreed as to their general mode of 

 formation from the particular cells, each constituent cell which 

 forms the stoma becoming divided into two or four stomatal cells, 

 according to the usual mode of cell-division, as already de- 

 scribed (see p. 732) ; these iiltimately separate from each other in 

 the line of partition so as to leave an orifice or interspace be- 

 tween, bordered by two or four stomatal cells respectively. 



5. FrxcTioxs of the Appe>.-dages of the Epioeemis. — 

 Hairs and their modifications appear to be designed to protect 

 the epidermis and parts beneath from injury from cold and other 

 external influences, hence we find young buds, &e., frequently 

 coated with hairs. Hairs also appear in certain instances, at 

 least to some extent, to absorb tiuid matters from the atmo- 

 sphere and soil ; whilst in other cases, they serve to assist the 

 epidermis in restraining exhalation, and we find, accordingly, that 

 plants which are densely coated with them are well adapted to 

 grow in dry arid situations, and to sustain without injury a 

 season of di-ought. 



Glands are those organs which contain some of the peculiar 

 secretions of the plant. These secretions are either permanently 

 stored up in them, or excreted. 



6. Functions of the Inteecellulae System. — The intercel- 

 lular canals, except in those cases in which the tissues of the plant 

 are gorged with sap, as in the spring of the year, are filled with 

 air, ancl the especial function which they perform is, to allow a 

 communication between the external air and the contents of the 

 internal tissues by virtue of the laws regulating the diffusion of 

 gases. They also facilitate exhalation of liquid, matters by their 

 connection wdth the stomata. The intercellular spaces are also, 

 in most cases, filled with air ; while the air-eel's and cavities, as 

 their names imply, are in like manner filled with aeriform 

 matters, and in water plants are especially designed to diminish 

 the specific gravity of the parts in which they are found, and thus 

 to enable them to float readily. The receptacles of secretion, as 

 their name implies, contain the peculiar secretions of certain 

 plants, and are closely allied in their nature to glands. (See p. 62.) 



Section 2. — Physiology of the Organs of Nuteition or 

 Vegetation. 



1. Of the Eoot oe Descending Axis. — The offices performed 

 by the root are: — 1. To fix the plant firmly in the earth or to 

 the substance upon which it grows, or in some aquatic plants, to 



