60 TYPES OF BRITISH PLANTS 
carbonic acid is kept as far as possible in order to make 
the liquor frothy. It does occasionally happen that the 
yeast plant goes on working longer than it ought, 
steadily making alcohol, and some bottled ginger-beer 
is a very doubtful temperance drink. The yeast plant 
itself is very simple in structure; it consists either of 
a single cell, or a branching chain of loosely-connected 
cells, which easily become detached from one another. 
Reproduction is very simple, and is produced by “ bud- 
ding,” that is to say, starting with our single cell, the 
first change is a bulge in the side of the oval wall. This 
bulge increases, some of the protoplasm flowing into 
it, and after a time the opening is walled off, and the 
new cell begins to bud, sometimes breaking away from 
its parent, and sometimes preserving the bond of union, 
though certainly enjoying entire home rule. The various 
yeast plants, some sixty in number, all work in much 
the same way, though according to the medium chosen, 
grape juice or barm and so on, they produce slightly 
different flavours. 
You will remember that the great test of the progress 
of a plant up the scale of vegetable society is the 
question, “How far has it adopted division of labour, 
especially in the matter of reproduction?” Such fungi 
as we have hitherto described are very simple, but when 
we come to higher forms, such as moulds, toadstools, and 
so on, we find there are traces of advance. In the first 
place, special structures are set aside for the purposes 
of a new family; and, in the second, we sometimes find 
a double set of machines, differing from one another, 
and corresponding to the stamens and pistils of flowering 
plants, or the father and mother pits of alge. But the 
curious thing is that we find both methods at work upon 
