840 ORIGIN OF SPECIES. 



and some climbing Palms like Calamus, &c., whose long shoots spread over 

 neighbouring plants and are supported by them, their hooked prickles and other 

 similar contrivances assisting in this. 



It is of service to many plants in the struggle for existence that they should 

 keep firm possession of the piece of ground they have once occupied, without 

 formino- for this purpose large woody masses, like trees and shrubs. The under- 

 ground parts of such plants are perennial, and they send up separate shoots in each 

 veo-etative period to be exposed to the light and air where they will be able to 

 assimilate, to produce flowers, and to scatter their seeds. This persistence of the 

 underground parts has the advantage that the plant, although it assimilates and 

 grows only at particular times of the year, is not compelled to seek each year, like 

 annual plants, a new locality in which its seeds may germinate. The collection of 

 reserve food-materials underground gives strength to the plant ; it developes its buds 

 beneath the soil to such an extent that at the right time they can grow up quickly 

 at the expense of the rich supply of food. Every year very strong shoots are put 

 forth, while in annual plants a number of feeble seedlings perish annually before 

 some of them attain sufficient strength to protect themselves from the shade and 

 humidity to which their neighbours subject them. Plants whose underground parts 

 are perennial have in particular the power of resisting long and severe frost and the 

 greatest variations of temperature, because these only penetrate slowly beneath the 

 soil. It is for this reason that so large a number of Alpine and Arctic plants belong 

 to this class. They are also able to grow in localities which are much too dry for 

 the germination of the seeds of annual plants, because moisture is retained at a 

 great depth for a longer period than near the surface. Numerous other advan- 

 tages might also be mentioned which are of course secured to annual plants by 

 other adaptations \ 



This permanence of the underground parts is attained in the greatest variety of 

 ways. Sometimes the plant possesses slender creeping underground shoots in which 

 the reserve food-materials are collected and which themselves rise above the surface 

 at a particular time, as in many Grasses ; or sometimes the leafy stems are developed 

 from lateral buds, as in Equisetum ; or there are thick stout stems from which shoots 

 appear each year at the same place. In some cases the whole plant is annually 

 renewed; all the parts which existed the previous year die off, and a complete 

 rejuvenescence of the individual is accomplished underground. In the potato and 

 artichoke only the apical parts of the underground lateral shoots swollen into tubers 

 remain over till the next year, the whole of the rest of the plant having perished. In 

 many of our native Orchids the rejuvenescence takes place in a similar way (see 

 p. 199 and fig. 150); and one of the most interesting cases of annual rejuvenes- 

 cence occurs in Colchicum auiumnale (see fig. 391, p. 545). In these cases, with 

 the exception of the Orchids, the reserve food materials accumulate in underground 

 parts of the axis ; in other cases this takes place in the swollen roots, which remain 

 in connection with the underground part of the stem that bears the new buds, as 



^ [This subject — and especially the relation of peculiar habits of life to the power of resisting 

 great cold — is very fully discussed in Kerner's treatise Die Abhangigkeit der Pflanzengestalt von 

 Klima und Boden, Innsbruck, 1869. — Ed.] 



