STEMS 



15 



23. Thorns, spines, prickles, and hairs. — Stems are sometimes 

 much reduced in size, modified in appearance, and take on new 

 functions. Thus, in wild pears and plums some branches are 

 reduced to small, hard, sharp points known as thorns or spines. 

 That these are modified branches is plain from the fact that they 

 arise in the axils of leaves 

 and show all gradations 

 from spines to spine-like 

 branches, often bearing- 

 leaves or lateral buds. 

 The prickles of brambles 

 and gooseberries, often 

 wrongly called thorns, are 

 not modified branches but 

 excrescences of the bark. 

 The stiff prickles of black- 

 berries may help to sup- 

 port the plant. 



In place of thorns or 

 spines, or possibly with 

 them, some plants have on 

 stems or twigs fine soft 

 hairs or down in which 

 case the}^ are said to be hairy or pubescent. Twigs of some 

 pome-fruits at some stages of gi^owth are very pubescent and 

 later may lose the hairs. Presence or absence of armament, 

 that is of thorns and spines and of bristles, is of capital im- 

 portance in distinguishing species of brambles and of bush- 

 fruits, nearly all of which are prickly or thorny or both with 

 great variations in the organs. An occasional species is thorn- 

 less or unarmed. 



In some species the thorns and prickles are of equal size and 

 of the same form ; in others, they are of different size and form. 

 In the European raspberry the prickles are slender and nearly 

 straight ; in the American red raspberry, they are much stouter ; 

 in blackberries and dewberries they are usually stout and curved. 

 Spines and thorns on some fruits are arranged in regular order 

 and in others are not. Some species, as the American red rasp- 

 berries, bear gland-tipped bristles or hairs on flowering shoots. 



Fig. 8. Fruiting shoot of Vitis La- 

 brusca showing continuous tendrils. 



