266 THE SEED PLANTS : FORMS AND LIFE HISTORIES 



when the aerial shoots have died ; and they store food 

 from which the next season's growth is started. 

 Primarily these underground shoots are thus a means 

 of perennation. Many plants which flower in early 

 spring have bulbs, corms or rhizomes,, and are able to 

 make new growth quickly because of the great store 

 of organic food at their immediate disposal. 



When a rhizome branches and the old parts die off 

 so that the branches are separated, the plant is pro- 

 pagated vegetatively, i.e. th& number of individuals is 

 multiplied by purely vegetative means, just as we 

 saw happened in Mosses and Liverworts, and as happens 

 also in Spirogyra when the ceU thread, after increasing 

 in length by cell division and growth, splits into lengths, 

 and produces a corresponding number of new plants. 



It is essentially the same proce&s when a plant 

 produces many tubers, or a Crocus corm several aerial 

 shoots, at the base of each of which a new corm is 

 formed, or when a bulb produces several new lateral 

 bulbs. Though the plant is thus multiplied, the new 

 individuals are developed close together, and compete 

 for the same space, light and food — the species is not 

 dispersed. But dispersal occurs, though gradually, 

 if the underground stems spread widely, producing 

 new roots and new aerial shoots from their tips. The 

 advantage of the bulb or corm is that it is better pro- 

 tected from desiccation than many rhizomes, and is 

 thus particularly well adapted to plants living in 

 climates with a very dry season. South Africa, for 

 instance, and the Mediterranean region have a large 

 number of bulb and corm producing species. The 

 British climate has no very dry season, and there are 

 many more rhizomatous plants which are able to 

 stand our comparatively mild winters perfectly well, 



