756 PHYSIOLOGY. 



pansion ; and in the second, by the development of antheridia 

 and archegonia upon the surface of this pro-thallus, by the ac- 

 tion of which there is ultimately produced a new plant, resem- 

 bling in every respect the one from which the spore was ori- 

 ginally derived. Hence Ferns and Horse-tails exhibit what has 

 been termed alternation of generations. 



2. Keproduction of Phanerogamous or Cotyledonous 

 Plants. — In all the plants belonging to this division of the 

 vegetable kingdom the male apparatus is represented by one or 

 more stamens, each of which essentially consists of an anther 

 enclosing pollen-grains (Jig. 423, p); and the female, by one or 

 more carpels (figs. 711 — 713), in or upon which (fig. 709), one 

 or more ovules are formed. When the ovules are enclosed in 

 an ovary, the plants to which they belong are called Angio- 

 spermous ; but when they are only placed upon metamorphosed 

 leaves or open carpels, the plants are said to be Gymyiospermous. 

 In the plants of both these divisions of the vegetable kingdom, 

 the ovules by the action of the pollen are developed into per- 

 fect seeds whilst connected to their parent, — the distinguishing 

 character of that seed from a spore being the presence of a 

 rudimentary plant called the embryo. The modes in which 

 reproduction takes place, and the after development of the 

 embryo, differ in several important particulars in Gymno- 

 spermous and Angiospermous plants; hence it is necessary to 

 describe them separately. 



1. Reproduction of Gymnospermia. — We have already given a 

 general description of the pollen and ovules, but as these 

 structures present certain differences in the Gymnospermia 

 from those found in the Angiospermia, it will be necessary for 

 us to allude to such peculiarities before describing the actual 

 process of reproduction. 



The pollen of the Phanerogamia generally, — that is of the 

 Angiospermia, consists, as Ave have seen (p. 359), of a cell 

 containing a matter called the fovilla, and having a wall which 

 is usually composed of two coats, the outer of which is termed 

 the eztine, and which possesses one or more pores (fig. 556), 

 or slits (^^s. 554 and 555), or both; and the inner, called 

 the intine, which is destitute of any pores or slits, and con- 

 sequently forms a completely closed membrane. Each pollen- 

 grain of the Angiospermia is thus seen to be a simple cell. 

 In the Gymnospermia, on the contrary, the pollen-grains are 

 not simple cells, but they contain other small cells, which 

 adhere to the inside of the internal membrane close to the point 

 where the external membrane presents a slit. 



The ovules of the Gymnospermia, excluding those of the Gne- 

 taceaj wliich require further investigation, consist of a nucleus 

 (fig. 1109, a), enclosed by a single coat, and with a large mi- 

 cropyle, m. Before the contact of the pollen with the micro- 



