ECOLOGICAL 185 



{b) On a second grade may be grouped all those cases where 

 multicellular plants multiply vegetatively, without specialised 

 reproductive cells, and therefore without involving any process 

 comparable to fertilisation. Thus a mould may spread by means of 

 rapidly growing, easily broken, threads (mycelial hyphse) over a 

 saucer of paste, or over the grass on the links, there forming "fairy 

 rings" by dying away internally as they extend externally. At a 

 higher level there may be multiplication by breakage of the long- 

 branched protonema threads which develop from the spores of 

 mosses; and very frequent is the multiplication of liverworts by 

 means of buds or gemmae, of which there is a gradation from one- 

 celled to many-celled forms. We have noticed elsewhere the peculiar 

 case of the soredia of lichens. Many plants which have specialised 

 sex-cells or less specialised spore-cells may also retain a capacity 

 for vegetative multiplication in natural conditions, as is illustrated 

 by the duckweeds (the minutest of Flowering Plants), or by the 

 bulbils that fall from the axils of the Tiger Lily. The most obvious 

 limitation to reproduction by means of detached pieces is that the 

 range of dispersal is not likely to be great, whereas spores and 

 seeds, being either minute or specially adapted, can be carried for 

 great distances by the wind, by water-currents, and by animals. 



(c) The third mode of multiplication in plants is by means of 

 special reproductive units, which may or may not require fertilisa- 

 tion. When the unit does not require fertilisation, it is called a spore; 

 but it is obviously difficult to draw a firm line between this and a 

 unicellular gemma, as seen, for instance, in some liverworts. More- 

 over, in most cases the life-story is complicated, as in Liverworts, 

 Mosses, Ferns, Horsetails, and higher plants, by the occurrence of 

 two generations — one, the sporophyte, producing germ-cells (spores) 

 which do not require fertilisation; and the other, the gametophyie. 

 producing dimorphic germ-cells (egg-cells and sperm-cells) in which 

 fertilisation must occur. 



It seems clearest to rank multicellular plants that multiply by 

 spores not requiring fertilisation along with those whose dimorphic 

 spores are definitely sex-cells and require fertilisation. For (i) there 

 is a deep difference between multiplication by spores and multipli- 

 cation by means of detached vegetative pieces, like gemmae; (2) 

 among the Algae there is often a fusion of spores (e.g. Ulothrix), 

 although it is not possible to distinguish the one as male and the 

 other as female; and yet in other Alga: (e.g. Fucus), the dimorphism 

 is pronounced, and the one set must be called egg-cells and the other 

 sperm-cells ; (3) there is a gradation between {a) independent spores, 

 (6) uniform spores that unite in pairs (isogamy), (c) dimorphic 

 spores that develop into two kinds of gametophytes (male and 

 female), and {d) those that develop into one kind of gametophyte 

 (with both male and female organs). In a typical non seeding higher 



