METAMORPHOSED SHOOTS 



37 



render it a matter of difficulty to recognise them as such. 

 For instance, stems may assume the appearance of leaves; 

 stems, leaves, or parts of them, may appear in the form 

 of spmes or of tendrils. It is mainly by the study of their 

 arrangement that we can recognise as such these meta- 

 morphosed stems and leaves, and can distinguish them from 

 " subsidiary outgrowths." 



SPINES, THORNS, AND PRICKLES. 



Many plants are armed with sharp-pointed woody structures, 

 which may represent stems, leaves, or subsidiary outgrowths. 



(i.) Stem-spines. — The 

 spines of the Hawthorn 

 (fig. 58) occupy the po- 

 sition of branches, for 

 they stand in the axils of 

 leaves ; they bear small 

 leaves, whicli soon fall 

 off. These spines, there- 

 fore, represent stems of 

 definite growth, whose 

 growing points become 

 hard and woody. Certain 

 branches of the Pear-tree 

 often end in spines, 

 which, therefore, are 

 metamorphosed stems. 



(ii.) Leaf-spines. — The 

 leaves of Thistles and of the Holly have spinose outgrowths, 

 which are obviously portions of the lamina. The Barberry- 

 plant has branched spines, in whose axils branches arise : 

 hence they are metamorphosed leaves. This view is confirmed 

 by the fact that on a stem of the Barberry it is often possible 

 to see all the transition stages between the gr'een leaves and 

 the branched spines. Each foliage-leaf of many Acacias has 

 two spines occupying the position of the two stipules : therefore 

 the spines are metamorphosed stipules. 



The Common Furze or Gorse {Ulex europaa) has thorns, 

 some of which possess axillary buds, and others of which 

 terminate stems. Thus in this plant both leaves and stems 

 have changed to form spines. 



Fig. 58. — Shoot of Hawthorn with a stem-spine 

 (rf) in the axil of a leaf. « represents stipules ; 

 b denotes the lamina. (After Dennert.) 



