THE THALLOPHYTE DIVISION 511 
been a very successful type of structure except in cases where some 
adventitious means of mechanical support could be depended 
upon, or, as in Mucor, dispensed with because of exceptionally 
favorable surroundings. But at best the utmost limit of size in a 
single cell is soon reached, and all large plants and animals consist 
of innumerable, minute cells. Muinuteness of the cell permits as a 
rule more rapid multiplication, and whether the cells be distinct 
individuals or the subindividual units of a body-community, 
minuteness facilitates taking advantage most promptly of all the 
food available. This principle is strikingly exemplified in- bacteria 
which are at once the smallest organisms known, and the ones ¢ca- 
pable of most rapid reproduction. Bacteria have been observed to 
divide at intervals of about a quarter of an hour. At this rate the 
progeny of one individual would be many millions in a single day. 
Most plants with a relatively large thallus reproduce vegetatively 
by setting free minute bits of their protoplasm. Thus most of the 
aquatic forms, e. g., Ulothrix, Coleochete, Laminaria, and Sapro- 
legnia convert certain of their protoplasts into swarm-spores resem- 
bling motile unicellular microbes; while the aerial Zygomycetes, 
Ascomycetes, and Basidiomycetes produce dust-spores. Both 
swarm-spores and dust-spores besides being quickly formed and 
readily set free, have the further advantage from their minuteness 
of being easily and widely dispersed by currents of water or air, 
just as was the case with the two halves of the ancestral fission- 
plants. The larger thallophytes throughout their evolution have 
thus retained, in their formation of minute non-sexual spores, the 
most primitive method of reproduction, while vegetating by a 
single enlarged cell or by a multicellular thallus. 
An obvious limitation of this primitive, rapid method of repro- 
duction lies in the fact that it depends upon a continuance of favor- 
able conditions for its success in perpetuating the species, whereas 
in nature such conditions are often suspended through periods of 
adversity. A very simple organism like Chroécoccus living under 
fairly uniform conditions can bide its time through seasons of cold 
and drought and resume its very moderate activity when warmth 
and moisture return. But the chances of injury are decreased and 
the power of taking prompt advantage of every favorable oppor- 
tunity to grow, retained, if at the approach of winter, for example, 
the tender protoplasts harden by getting rid of superfluous water, 
become invested by a thicker, firmer wall, and store up what food 
they can instead of spending it all in immediate growth and re- 
production. A resting spore such as that of Nostoc thus provides 
for the future. 
An improvement upon this simplest type of resting spore is the 
zygospore in which two protoplasts co-operate to form a single, 
relatively large cell, well protected and richly stored with food for 
use at the time of germination. Even in the most primitive cases 
of co-operative provision for the welfare of offspring, a far-reaching 
