LEAF STRUCTURE. 



173 



taking in of oxygen and its combination with other 

 substances while in the lungs and the liberation of 

 substances thus formed constitute respiration in ani- 

 mals. So with plants ; they suck or draw in air 

 through openings in the epidermis already described, 

 and when it is discharged it is found to be changed in 

 character, having been robbed of its oxygen or of its 

 carbon dioxide. The oxygen of the air while among 



8T 



528 



527, Is the section of a young stem or branch showing, at p, the pith; a, vascular-fibro bundle, passing 

 off from the stem to form the leaf-stalk and frame- work of the blade; d, the swelling just below the foot of 

 the stalk; I, the base of the footstalk; 6, the axillary bud; c, the articulation or point where the leaf-stalk 

 is attached to the branch or stem. 528, Magnified section of a leaf perpendicular to its surface; P, hair on 

 the upper surface; ST, stoma; Es, epidermis of the upper surface made up of plate cells ( 391); Ps, oval 

 cells closely packed with longer ones perpendicular to the epidermis; M, interspace beneath the stoma; 

 i, interspaces among the irregular shaped, loosely packed cells of the lower stratum Pi; Fv, cross-section 

 of fibro-vascular bundle: JSi, lower epidermis with hairs. (See Figs. 489 to 500.) 



the tissues unites with substances found there, and 

 new material for plant growth is thus formed ; in the 

 night carbon dioxide is breathed out. It has been 

 shown by experiment that air is not only required for 

 the health of plants, but that they can not exist with- 

 out it ; for when placed in a vacuum, they invariably 

 perish. Respiration is therefore' necessary to the life 

 of plants as well as to animal life. . 



428. Breathing goes on in all parts of plants ex- 

 posed to the air, at night as well as in the daytime; 



