THE THALLOPHYTE D I \' I SIGN 511 



been a very successful tyi)e of structure except in cases where some 

 adventitious means of mechanical suj)j)ort could be depended 

 upon, or, as in Mucor, disjiensed with because of exceptionally 

 favorable sui-roundings. But at best the utmost limit of size in a 

 single cell is soon, reached, and all large i:)lants and animals consist 

 of innumerable, minute cells. Minuteness 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 vegetativeh'' 

 by setting free minute bits of their protoplasm. Thus most of the 

 aquatic forms, e. g., Ulothrix, Coleocha^te, Laminaria, and Sapro- 

 legnia convert certain of their protoplasts into swarm-spores resem- 

 bling motile unicellular microbes; Avhile 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 easilj^ 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 thalloph.ytes 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 primiti\e, 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 Chroococcus 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 



