CUTICLES AND POKES OF LEAVES. 73 



various climates in which these plants have been respectively 

 destined to flourish, for the thin cuticle of a species indigenous 

 to temperate climates, would not have afforded a sufficient pro- 

 tection to the interior structure had it been exposed to the 

 vertical rays of a tropical sun, whilst the diminished heat of 

 this country would scarcely overcome the resistance of the 

 dense and non-conducting tegument of a species formed to 

 exist in tropical climates. Nor must we forget that the thick- 

 ness of the badly-conducting cuticle serves also to protect the 

 leaves of the equinoctial plants against the great difference of 

 temperature which frequently exists between the heat of the 

 day and the chilly coolness of the night, a difference much 

 greater than that which takes place in the temperate regions. 



As the cuticle is impermeable to air, it may well be asked 

 how the leaves are able to perform their respiratory functions ; 

 but the enigma is soon 

 solved on examining a leaf 

 through a powerful micro- 

 scope, for then the cuticle 

 is seen to be pierced with 

 numerous pores, or sto- 

 mata, leading into lacunce 

 or air - chambers, small 



onpn srmppcj m'tnatprl in HIP Vertical section of portion of a leaf of Iris 

 Open Spaces Situated m tne Germamca, taken in a transverse direction. 



green Cellular tissue Of the , cells of the cuticle ; 6.6, cells at the sides of the 

 , , , . rv 1 . stomata ; c.c, small green cells placed within these ; 



leaf, and thus affording a d-d, openings of the stomata; e,e, lacuna? or air 



cells ; /./, cells of the parenchyma. 



passage to the atmosphere. 



In general, the stomata are not so numerous on the upper as 

 on the under surface of the leaf; frequently even, as for instance 

 in the oak, the beech, the birch, and the alder, they are en- 

 tirely confined to the latter. In the erect leaves of the grasses 

 they are about equal on both sides ; in leaves floating on the sur- 

 face of the water they are found only on that side which is exposed 

 to the air ; and in submerged leaves they are, with but few excep- 

 tions, completely wanting. Generally, they are least numerous 

 in succulent plants, whose moisture is destined to be retained 

 in the system ; whilst they abound most in those species in which 

 a rapid absorption and exhalation of the fluids takes place. In 

 the Hydrangea, for instance, there are no less than 16,000 

 stomata in every square inch of the under surface of the leaves ; 



