I08 PLANT LIFE. 



off, but the central mid-rib remains as a strong 

 hard spine. These withered midrib-spines entirely 

 cover and protect the leaflets on the younger leaves. 

 The common Acacias of the desert have usually very 

 long spines, which are formed from the stipules of the 

 leaves ; the same arrangement is found in False Acacia 

 {Robinid), which is not uncommon in this country in 

 shrubberies. They are, in this last case, small and 

 curved, so that an animal must find it difficult to strip 

 off the young leaves which appear from the bud just 

 above. The Barberry has spines which are produced 

 from a whole leaf; they protect the bunch of small 

 tender leaves which develop just above them. 



The thorns of the Sloe, Hawthorn and Whin are modified 

 branches (the leaves in the last-mentioned case are also 

 spiny). All these plants grow in positions where they are 

 specially open to attack by grazing animals. The Bramble 

 forms a thicket exceedingly difficult to penetrate because 

 the strongly armed branches root at the tips. Both in 

 the Bramble and in the Rose the prickles are neither 

 leaves nor branches, but excrescences which are produced 

 without any relation to the leaf Both Rose and Bramble 

 use their prickles, partly for defence and partly to assist 

 them in climbing. Their long branches straggle over 

 those of taller plants and hook themselves upon them. A 

 very remarkable prickly form is a Wild Yam {Dioscorea 

 prehensilis). The plant grows in the Taru Desert of East 

 Africa where wild boars of a special kind occur. The 

 plant is about lo feet in height and looks at first sight 

 like an immense arbour covered by an intricate network 

 of strongly prickled roots. In the centre there is a 

 fleshy root-tuber full of food material. There are very 

 few cases of spiny roots ; but some Iridaceae and also 

 certain Palms possess them. 



