OP BOTANY. XXXV 



510. The number of cotyledons varies from one to several. The most common 

 number is either one or two. In the latter case, they are always directly opposite 

 each other. 



511. Plants that have but one cotyledon, or if two, then the cotyledons alternate 

 with each other, are called Monocotyledonous. 



512. Plants that have two opposite each other, or a greater number placed in a 

 whorl, are called Dicotyledonous. 



513. Endogenous plants are monocotyledonous. 



514. Exogenous plants are dicotyledonous. 



515. Plants that have no cotyledons are said to be Acotyledonous. But this 

 term is usually applied only to cellular plants, which, having no stamina and pistils, 

 can have no seeds (470,393). Those seeds of flowering plants, which appear to have 

 no cotyledons, owe their appearance to the cotyledons being consolidated ; Ex. 

 Cuscuta, Lecythis, Olynthia. 



516. The plumula is very often latent, until it is called into action by the germina- 

 tion of the seed. Sometimes it is undistinguishable from the cotyledons ; sometimes 

 it is highly developed, and lies in a furrow of the cotyledon ; Ex. Maize. In the 

 monocotyledonous embryo it frequently happens that the plumula is rolled up in the 

 cotyledon, the margins of which grow together, so that the whole embryo forms one 

 uniform mass ; but as soon as germination commences the parts separate. 



517. The radicle elongates downwards, either directly from the base of the em- 

 bryo, or after previously rupturing the integument of the base. Plants with the 

 first character are called Exorhiz.e ; with the second, Endorhiz^:. 



518. The endorhizous embryo is very common in monocotyledons ; the exorhizous 

 in dicotyledons. The characters of the radicle are, however, far from being con- 

 stant in those great divisions of the vegetable kingdom. 



519. The direction of the embryo, with respect to the seed, will depend upon the 

 relation that the integuments, the raphe, chalaza, hilum, and micropyle, bear to each 

 other. 



520. If the nucleus be inverted, the embryo will be erect, or or thotropous. Ex. 

 Apple. 



521. If the nucleus be erect, the embryo will be inverted, or antitropous. Ex. 

 Nettle. 



522. If the micropyle is at neither end of the seed, the embryo will be neither 

 erect nor inverted, but will be in a more or less oblique direction with respect to the 

 seed ; Ex. Primrose ; and is said to be heterotropous. 



523. When the seed is called into action, germination takes place. The juices 

 of which before were insipid, immediately afterwards abound with sugar ; Ex. Bar- 

 ley ; and growth commences. 



524. This growth is in the first instance caused by the absorption of water by the 

 seed, and by the expulsion of superfluous carbon by the cotyledons, in the form of 

 carbonic acid gas. 



XVIII. FLOWERLESS PLANTS. 



525. Many plants, not being increased by seeds, the result of the mutual action of 

 the stamina and pistils (470), are flowerless, or destitute of organs of fructification. 



526. Such are propagated by what are called organs of reproduction, which have 

 no other analogy with the organs of fructification than that both perpetuate the 

 species. 



527. The reproductive organs of flowerless plants vary according to the tribes of 

 that division of the vegetable kingdom, and have so little relation to each other, 

 that each principal tribe may be said to have its own peculiar method of propa- 

 gation. 



528. The principal tribe are Ferns, (529), Mosses (535), Lichens (541), Alga 

 (542), and Fungi (543). 



529. Ferns are increased by little bodies called sporules, enclosed within cases 

 named thecce, which often grow in clusters or sori, from the veins of the under 

 sides of the leaves, or from beneath the cuticle. The latter, when it encloses the 

 thecce, is termed the indusium. 



530. The indusium separates from the leaf in various ways, in consequence of 

 the growth of the thecae beneath it. 



531. The thecal have frequently a stalk which passes up one side, and finally, 

 curving with their curvature, disappears on the opposite side. 



