938 ORIGIN OF SPECIES, 



If looked at merely superficially, the mode in which the Virginian Creeper 

 climbs up rocks, walls, and thick trees, presents a certain resemblance to the climb- 

 ing of the Ivy ; but in fact the adaptations of the two are altogether different. It 

 has already been shown how negative heliotropism causes the leafy branches of the 

 Ivy to become closely pressed to the support, and how the summit of the branch at 

 first exhibits slight positive heliotropism, so that the slight convexity is in contact 

 with the support. At this point of pressure rows of aerial roots afterwards arise (not 

 in consequence of pressure, for they make their appearance also on branches which 

 hang free) which apply themselves to the inequalities of the bark of the tree or of 

 the rock which serves as a support, and thus fix the Ivy-stem to it. Other weak- 

 stemmed plants attain the same object (that of elevating their assimilating and 

 flowering shoots) by apparently much simpler means, as the Bramble, Rose, and 

 some cHmbing Palms hke Calamus, &c., whose long shoots spread over neighbour- 

 ing plants and are supported by them, their hooked prickles and other similar con- 

 trivances 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 

 forming 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 

 vegetative 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 material. 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 advantages might 

 also be mentioned which are compensated for in 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 



* [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.] 



