226 



HUTCHINSON'S POPULAR BOTANY 



seasons), a tough leathery membrane to retard evaporation, and formidable- 

 spines as a protection from browsing cattle. Sometimes these spines get 

 into the breasts of buffaloes and other large animals, causing inflammation 

 and even death, and the wild asses of the desert are often lamed by them. 

 Compare the stems of the two species of African Spurge (Euphorbia 

 grandicornis and E. abyssinica) shown in fig. 286 with the slender European 

 species (Euphorbia splnosa). 



Weak-stemmed plants, which object to the low earth- 

 trailing life that satisfies a Strawberry-plant or Creeping 

 Buttercup, resort to all manners of devices in order to 

 grow upwards. Thus the Ivy (Hedera helix) climbs by 

 means of its short and multitudinous aerial roots it is. 

 a root-climber ; the Bramble (Rubus fruticosus) and the- 

 Wild Rose (Rosa arvensis] hook climbers develop prickles 

 on their stems, whose curved points enable the plant to 

 cling to whatever will help its ascent ; the Traveller's 

 Joy (Clematis vitalba) and Garden Nasturtium (Tropceolum- 

 majus] leaf climbers both gain the desired end by means 

 of their leaf-stalks, which they twist round the nearest 

 support; the Vine (Vitis vinifera) and Virginia Creeper 

 ! (Vitis quinquefolia] mount upwards by help of tendrils, 



J which, in the plants named, are metamorphosed branches 



!' with adhesive discs, but in others as the Sweet Pea. 



(Lathyrus odoratus), Yellow Vetchling (L. aphaca}, Smilax, 

 and (possibly) White Bryony (Bryonia dioica) are meta- 

 morphosed leaves and stipules.* Ercilla volubilis, a 

 Chilian climber, attaches itself to any available support 

 by means of adhesive discs borne directly upon the 

 branches just above the axils of the leaves. Lastly, 

 the stem itself may entwine the supporting object, when 

 its spiral course is in some plants always to the left (e.g~ 

 the Convolvulus, Black Bryony, and the Scarlet Runner 

 Bean), in others always to the right, as the Hop (Humidus 

 lupv/lus) and Honeysuckle, albeit external conditions have 

 no influence on the maintenance of these directions. The 

 climbing proclivities of the Hop are greatly facilitated 

 by the development of innumerable anvil-shaped hooks- 

 on the ridges of its hexagonal branches. 

 Plants whose shoots twine always to the right i.e. clockwise are- 

 called dextrorse climbers ; while those whose shoots take the opposite direc- 

 tion i.e. counter-clockwise are described as sinistrorse climbers. " It is a. 

 matter of indifference to the direction of these movements/' says Kerner r 



* Some are of opinion that the so-called climbing stipules of the Bryony are really 

 extra-axillary branches. 



FIG. 281. CLUB 



RUSH (Scirpus 



pungens), 



Showing triangular stem 

 or calamus ; also flowers. 



