138 PROTECTION OF THE GKEEN TISSUE 



their leaves can be reached by grazing animals. 

 As soon as their twigs and branches are out of 

 reach of the jaws of the beasts, they develop no 

 thorns. The Holly is an example of this. The 

 leaves which deck the crown of the high tree- 

 trunks are entire and unarmed, while on the low 

 shrubs the margin of the leaves is drawn out into 

 sharp, spiny teeth. 1 



We may divide the weapons of the plant into 

 two classes, the first of which includes the forms 

 where the green tissue is protected by thorns and 

 prickles developed on the green parts themselves ; 

 and the second, those where other portions of the 

 plant are turned into arms to defend the unarmed 

 green parts. 



To the first class belong those leafless plants 

 which have developed green tissue in their stems 

 and twigs. It is true that the green branches of 



1 This is the subject of a little poem by Southey : 



" O Reader! hast thou ever stood to see 



The Holly-tree? 

 The eye that contemplates it will perceive 



Its glossy leaves; 



Ordered by an Intelligence so wise 

 As might confound an atheist's sophistries. 

 Below, a circling fence, its leaves are seen 



Wrinkled and keen; 

 No grazing cattle through their prickly round 



Can reach to wound; 



But, as they grow where nothing is to fear, 

 Smooth and unarmed their pointless leaves appear." 



