INTRODUCTION. 29 
spore-cases. The colour, no less than the form of these spores, is 
variable; they are usually pale brownish or yellowish, but they 
are sometimes green, and the tints of brown and -yellow are much 
varied. These organs differ obviously from seeds, in that they | 
consist merely of a homogeneous cellular mass. In true seeds 
the radicle or young root, and the plumule or young shoot, are 
present in the embryo, and are developed from determinate points ; 
but Fern spores, consisting merely of a small vesicle of cellular 
tissue—a vegetable cell, grow indifferently from any part of 
their surface, the parent cell becoming divided into others, which 
are again multiplied and enlarged, until a small green germinal 
scale, or primordial frond, is formed, and from this, in due 
time, the proper fronds are produced. The surface of the 
spores is sometimes smooth, sometimes tuberculated, or ‘even 
echinate. 
The germination of the spores of Ferns has lately excited much 
inquiry, the result of which leads to the inference that something 
like sexuality exists among all the higher groups of the Cryptoga- 
mous plants, a kind of fertilisation taking place on the prothallus or 
germ-frond, which in the Ferns, as already mentioned, takes the 
form of a small leafy scale. The balance of evidence is unques- 
tionably in favour of the existence in the Ferns of distinct sexes, and 
of a process of impregnation which gives rise to a new individual. 
It has also been ascertained that something like what in the animal 
kingdom is called an alternation of generations, takes place in the 
Ferns, the one complete generation consisting of the prothallus, 
which is developed from the spore, and bears the antheridia and 
pistillidia, through which fertilisation takes place; the other genera- 
tion, resulting from this act of fertilisation, being totally different, 
much more developed, and producing stems, fronds, and spores. The 
facts from which these inferences are drawn have been variously 
stated by different observers. The development of the spores, 

