178 Structural and Physiolog ical Bo tany ys c 
one another. For example, a generation of the kind a will _ 
produce one of the kind pz, and this again one of the kind a. 
This kind of development is called an alternation of genera-— 
tions. 
Processes of reproduction are, again, of two essentially 
different kinds. While one class or group of reproductive | 
cells are capable, without further preparation, of giving 
rise to a new plant, another class only have this power im-. 
parted to them by other cells which stand to them in a 
relation of contrast. ‘The former kind are termed in general 
asexual reproductive cells, the latter germ-cells or oospheres. 
Reproduction by asexual reproductive cells [asexual re- | 
production| occurs in a variety of modifications, some less, 
some more perfect, and is variously termed reproduction 
by spores, gemme, bulbtls, conidia, buds, gonidia, &c. 
The most common mode of multiplication in the lower 
plants is by means of sfores.' This term may, in general, 
be applied to all those simple reproductive cells which arise 
under the ordinary conditions of life. In by far the greater 
number of cases they are simple cells enclosed in a single 
or double cell-wall, and formed at particular places, very 
commonly in special receptacles to which the name sfo- 
rangia is given. ‘The great variety that prevails in the form, 
function, permanence, and mode of formation of spores has 
led to the use of a number of special terms, of which those 
in most common use will be hereafter mentioned and ex-. 
plained? (See eg. Figs. 393-403, 440, 450.) 
1 [The term ‘spore’ is unfortunately currently used in cryptogamic 
botany in the widest and vaguest sense. In Fungi, Mosses, and most 
Vascular Cryptogams, the spores are asexual reproductive cells in the 
sense here employed, as also are the ‘tetraspores’ of the Rhodosporez. 
But the ‘zygospores’ of Conjugate and the ‘resting-spores’ of some 
other Algze are the result of a process of union ; while the ‘spores’ of 
the Fucaceze are oospheres which require to be fertilised in order to 
enable them to germinate.—ED. | | 
2 [Special mention should be made, under the head of asexual repro- 
ductive cells, of zoospores or swarmspores, bodies found exclusively in 
Cellular Cryptogams, endowed with a remarkable spontaneous power of 
