450 ELEMENTARY BIOLOGY. [CHAP. 



gradually assumes a bilobed form, and becomes thickened 

 in its middle part by division of its cells in a direction 

 parallel to its surface. The thickened portion is termed 

 the cushion. Numerous colourless, unicellular root-hairs 

 are given off from the under surface of the little plant, 

 which is called a prothallus or prothallium, and attach it to 

 the surface on which it grows. 



The prothallus attains no higher development than this, 

 and does not directly grow into a fern such as that in which 

 the spores took their origin ; but, after a time, rounded or 

 ovoidal elevations are developed, by the outgrowth and 

 division of the cells which form its under surface. These 

 bodies are of two kinds, the antheridia and archegonia. In 

 the former, which may be developed on any part of the 

 under side of the prothallus, the nucleus of each of the 

 cells contained in their interior is converted into a sperma- 

 tozoid somewhat similar to that of Chara, but provided 

 with many more cilia. The antheridium bursts, and the 

 spermatozoids set free from their containing cells are pro- 

 pelled through the moisture on the under surface of the 

 prothallus by their cilia. 



The processes of the second kind, the archegonia, acquire 

 a more cylindrical form. The outside of the organ is formed 

 of a single layer of cells, which are persistent, but those 

 which are situated in the axis of the archegonitim all dis- 

 appear with the exception of that which lies at the bottom 

 of its cavity. This is the ovum or oosphere, and when the 

 archegonium is fully formed, a canal leads from its summit 

 to this cell. The spermatozoids enter by this canal, and 

 impregnate the ovum. 



The ovum now begins to divide, and becomes converted 

 into eight cells, which give rise to the stem, leaf, root and 

 foot, of the fern-plant, the foot being the organ by which 



