38 BOTANY. 
the reproductive organs of cryptogamous plants, and now propose to combine 
these under one general head, with the .addition of some points hitherto 
omitted. In the caseo Ferns, reproduction is effected by means of spores, 
inclosed in cases named thece, which often form in clusters or sori on the 
under side of the leaves, or beneath the epidermis. This latter, when includ- 
ing the thecex, is called the indusiwm. The thece, or spore cases, have 
frequently a stalk passing up one side, and disappearing on the other; the 
point where this is attached is called the annulus. : 
Urw Mosses are increased by spores contained within an urn, placed 
at the apex of a seta or stalk, bearing on the summit a loose hood, called a 
calyptra, and closed by a lid or operculum. At the base of the spore case is 
sometimes found a tumor or struma, or an equal expansion, termed apophysis. 
The inside of the thece has a central axis, or columella ; and the orifice 
beneath the operculum is closed by teeth-like processes, or a membrane called 
peristome. 
Licnens are cellular expansions, consisting of a thallus, or combination 
of stems and leaves, upon which appear shields or apothecia. These are 
the reproductive organs, and consist of a margin inclosing a kernel or 
nucleus, in which are imbedded tubes containing sporules, and. termed 
asc. : 
In the highest forms of Funeats there are two kinds of organs: one, cys- 
tidia, conical naked elevations; the other, basidia, also conical elevations, but 
bearing spores on their apex. 7 
GENERAL CONSIDERATIONS WITH RESPECT TO PLANTS. 
A. Tue CHemicaL ConsTITUENTS oF PLANTs. 
Plants are composed of certain chemical elements, which are combined 
in various ways, so as to form either organic or inorganic compounds. The 
former are composed of carbon, oxygen, hydrogen, and nitrogen, with a 
certain proportion of sulphur, or phosphorus; the latter consist of various 
metallic bases, combined with metalloids and acids. Water is a chief con- 
stituent of plants, the amount being determinable with approximate accuracy, 
by drying the plant at a heat slightly exceeding that of boiling water, and 
estimating the loss of weight. When the plant is burned the organic con- 
stituents disappear, and the inorganic are left in the form of binary or ternary 
compounds. 
CaRBON is the most abundant constituent of the plant, forming a greater 
proportion of its mass than any other. When vegetable matter is heated 
without exposure to air, the carbon is left in the form of charcoal, of a black 
color and porous texture, retaining much of the original volume and shape. 
It exists in great quantity in the soil, but requires to be converted into 
carbonic acid before it can be taken up by the plant. Most of the carbon 
of the plant is derived from the decomposition of the carbonic acid of the 
38 
