THE EPIDERMIS. 



11 



this substance. In grinding, both of the upper layers of 

 cells, as far as c, are lost with the bran, and thus by far 

 the most nutritive portion of the grain is cast away. 

 This shows that the millstone is not only " a bad analyzer " 

 but a bad economist. The discovery of some more deli- 

 cate process of separating the bran from the flour, or at 

 least of extracting the gluten from the bran, so as to 

 make it available economically, is a subject of study for 

 the ingenious well worthy of attention. 



73. ALKALOIDS. These are possessed of the most 

 active properties, and many of them are virulent poisons. 

 Morphine, which is the chief anodyne principle in the 

 Poppy; Quinine, or the bitter-tonic principle of the 

 Peruvian Bark ; Strichnine, which is the terrible poison 

 of the Mandioc and Nux Vomica ; Nicotine, the poison 

 principle of Tobacco ; and Conine, that of the Poison 

 Hemlock, are all alkaloids. 



74. CRYSTALS. That plants contain certain earthy 

 matters is satisfactorily shown by the deposition of ashes 

 after burning. These are not always diffused in dismem- 

 bered particles, but sometimes they occur in the form of 

 crystals. At fig. 2 may be seen two of these crystalline 

 clusters, from cells of the Beet. The crystals are of 

 various forms, and are frequently needle-shaped ; some- 

 times deposited singly, at others in bundles of several 

 hundred together, as we see in fig. 5, which represents 

 several cells from the Dock, two of them containing such 

 bundles. The name of Raphides (a Greek word for 

 needles) was originally given exclusively to crystals of 

 this kind. It has since been extended to all such crys- 

 talline formations, though, properly speaking, the term 

 should be restricted to such as are needle-shaped. The 

 leaf of the Locust tree contains multitudes of crystals. 

 They are abundant in the root of Turkey Khubarb, and 

 in the Old-man Cactus they constitute about two thirds 

 of the dried tissue. In the bulbs of the Spanish Squill 

 and Onion, the numerous crystals may easily be sepa- 

 rated when the surrounding parts fall into decay, or by 

 boiling. 



75. SILEX. And not only are crystals deposited in 

 the cells, but silex, or the earth of flints (of which com- 

 mon sand is an impure condition), enters into the struc- 

 ture of the cell walls, thus strengthening and solidifying 

 the simple cellulose. Were it not for this, the slender 

 stalks of Rye, Wheat, and most other Grasses, could 

 not support themselves in an erect position. The tribe 

 of plants known as Horse-tails afford remarkable instances 



Grain of Eye. Alkaloids, what properties ? Instances. Crystals, what called ? 

 What forms? Where found? In what leaf abundant? Of what plant do they con- 

 stitute a large portion ? Silex, in what plants? What parts? Most remarkable 

 instance. 



of such depositions of earthy matter ; and to this cir- 

 cumstance the Scouring-rush owes its value in the country, 

 where it is sometimes used as an article for polishing 

 metals. 



76. But the most astonishing accumulation of mineral 

 matter in the vegetable form, is found in that large class 

 of Sea-weeds of the lowest type, which pass by insensible 

 gradations into animal organisms. In these, almost the 

 entire cell wall is composed of silex, which, as the plants 

 decay, is deposited at the bottom of the waters where 

 they are found ; and thus immense strata are sometimes 

 formed. 



CHAPTER VII. 



THE EPIDEEMIS. 



77. THE vegetable, like the animal form, is enveloped 

 in a membranous covering called the EPIDERMIS, or skin. 

 This envelope appears to the naked eye as a transparent 

 and colorless integument, without any organic character ; 

 but on examining it through a lens, we find that it is 

 composed of one or more layers of flattened cells. The 

 edge of these may be seen in the straight lines of cells 

 that bound the upper and under sides of the section of 

 Melon leaf, fig. 17. Every part of the plant which is 

 directly exposed to the air, except the stigma, is protected 

 by the epidermis. The peculiar office of this very curious 

 organ will be more distinctly treated in the anatomy of 

 the Leaf, where it may be again referred to as its true 

 place. At present, some of its modifications or appen- 

 dages will be defined. 



EPIDERMAL APPENDAGES. 



78. The chief of these are, Stomata, Hairs, Stings, 

 Prickles and Glands. 



79. STOMATA. These are orifices in the epidermis 

 of leaves and other green parts, by means of which a 

 communication is kept up between the external air and 

 the intercellular spaces. At fig. 6 is a transverse sec- 

 tion of the epidermal cells, with several oblong bodies, 

 which are the stomata. This organ is among the most 

 curious and wonderful contrivances in the vegetable 

 fabric, and a more complete account of it will be intro- 

 duced with the Leaf, to which it especially belongs. 



80. HAIRS. These very common appendages consist 

 of a protrusion of the epidermal cells into distinct points 



Subject of the chapter. What is that? How appear to the naked eye? 

 How under microscope ? Epidermal appendages. Stomata. Where found 

 Hairs. 



