REPRODUCTION AND DISPERSAL 809 



Auxospores. — In the diatoms vegetative reproduction tal^es place by longitudinal 

 splitting, but each new individual often is shorter than the last, because it is formed 

 within the old and rigid silicious wall. In time progressive diminution ceases, and 

 the protoplast escapes, whereupon it enlarges to the original size and again becomes 

 incased by rigid silicious walls. The enlarging protoplast is called an auxospore. 



The significance of vegetative reproduction. — Vegetative propagation 

 is the most universal kind of reproduction. Some plants (as the bac- 

 teria and lower algae) have no other kind, while very few plants {e.g. 

 some annuals, biennials, and trees) are altogether without it. Many 

 plants that are capable of producing spores or sex organs nevertheless 

 spread almost wholly by vegetative means; among such plants are many 

 mosses, some liverworts (as Limularia), and even some of the higher 

 plants (as the duckweeds). In far northern regions many plants are 

 said to reproduce as a rule only vegetatively, the summer being too short 

 for seed production. Even those plants that fruit regularly usually 

 spread much more by vegetative propagation than by spores or seeds. 

 Thus there can be no doubt that vegetative reproduction is the chief 

 factor in the maintenance of species and in the enlargement of their areas. 



The chief disadvantage associated with vegetative reproduction is that 

 propagules rarely are fitted for distant dispersal. Hence the invasion 

 of new areas by this means alone is slow; the ultimate estabhshment of 

 a species by vegetative reproduction in a distant region is even im- 

 possible unless favorable habitats are continuous, since propagules 

 rarely are able to cross barriers. For example, the rhizomes of meso- 

 phytes (such as the Solomon's seal and various ferns) are unable to 

 migrate over bodies of water or dry ridges, although such migration may 

 be accomplished quickly by most spores and by many seeds. Some- 

 times, however, propagules are true disseminules, notably among the 

 water plants, where portions of plants may become detached and float 

 for great distances, thus equaling seeds in mobility and in the wide- 

 spread invasion of new regions. Among land plants the distant dispersal 

 of vegetative disseminules is coniined chiefly to gemmae; the minute 

 gemmae of various liverworts may be scattered for some distance by 

 wind, and lichen soredia are scattered almost as effectively as are spores. 



Reproduction by asexually formed spores. — General characteristics. — 

 Asexually formed spores ' commonly are unicellular structures (multinu- 

 cleate in Vaucheria) , produced, as a rule, by specialized spore-bearing 

 organs (sporangia, etc. ; see Part I) . Generally they differ from gemmae 



• Often these are called for convenience asexual spores or simply spores. 



