ANATOMY OF THE LEAF, 27 



size varies; they frequently contain in their interior large spiral fibres coiled 

 up; and they are so numerous and strong in the leaves of the Bananas and 

 Plantains of India, that the natives draw them out by handfiiUs, and dry 

 them for tinder. There seems to be a universal tendency in all organized 

 masses to develop themselves in a spiral manner; and in respect to the arrange- 

 ment of leaves on the stem, its consideration is included in that department 

 of the study of Morphology, which has received the name of Phylhtaxis. 



Beside these spiral vessels, there is another class of vessels, called Laticiferous^ 

 because there circulates in them a peculiar milky 



fluid, called Latex. This stnicture is more abun- >\^:T:Tr77 ^■ 



dant in some plants than in others; this is es- ):/' 1-/^~~~1'' 



pecially the case in the Common Dandelion. l:L__^.-( /•/ 



Thus much for the vessels or vascular system 

 of the leaf; let me now direct particular attention 

 to the skin or covering of it: this is a more 

 interesting structure. 



The external covering, or Epidermis generally 

 consists of a single layer of transparent cells on '*^'*' ^ 



each side, which do not contain any of the gi'een gi'anular matter of 

 which we have spoken. It is remarkable for the number of openings or mouths 

 which it presents. 



The mouths, or Stomata, are formed by the apposition of two or more 

 cells, which have the appearance of lips. The lips close or open according to 

 the humidity or dryness of the atmosphere, and thus regulate the passage of 

 fluids to the leaves. They occur more generally upon the under than upon? 

 the upper side of the leaf, sometimes in equal numbers upon both. Their 

 number also varies in different plants. 



Many leaves are covered with hairs. These, hairs are simply prolongations 

 of cells from the general covering; but, like all other organs of the plant, 

 their external configuration is subject to the greatest variety of form. Hairs 

 are by no means of little importance. Cotton itself is manufiictured from the- 

 hairs surrounding the seeds of the Gossypium herhaceum, or common cotton- 

 plant of India. Many hairs secrete at their base an acrid irritating matter, 

 which, if applied to the skin, communicates a burning sensation, frequently 

 attended by great pain. Every one is familiar with the sting of the common 

 nettle. These stings are conical hairs, the apex or top of which is closed 

 by a globular or button-shaped projection, resembling a pin's head. The 

 slightest touch causes this little prominence to fall off, when at the same 

 time the sharp end of the hair immediately beneath pierces the skin, and the- 

 irritating fluid within is thrown out. 



Some hairs secrete oily, resinous, and adhesive matters; in the latter case,, 

 insects lighting upon the leaves stick there and die. By such means some 

 plants are preserved from the injurious attacks of insects, which would other- 

 wise have effected theii* destruction. Hairs also servo to protect certain plants 



