STRUCTURE.] SPINES BULBS. 171 



special power of extension in length. Under ordinary cir- 

 cumstances, the growing point clothes itself with leaves as it 

 advances, and then it becomes a branch ; but sometimes it 

 simply hardens as it grows, producing no leaves, but forming 

 a sharp conical projection called a spine, as in the Gleditschia, 

 the Sloe, &c. When formed it does not, however, consist of 

 cellular tissue alone ; on the contrary, it has the same general 

 internal structure as the perfect branches themselves. 



The spine must not be confounded with the prickle or 

 aculeus already described, (p. 164), from which it differs in 

 having a considerable quantity of woody tissue in its structure, 

 and in being as much in communication with the central 

 parts of a stem as branches themselves ; while prickles are 

 merely superficial concretions of hardened cellular tissue. 

 Spines occasionally, as in the Whitethorn, bear leaves; in 

 domesticated plants they often entirely disappear, as in the 

 Apple and Pear, the wild varieties of which are spiny, and 

 the cultivated ones spineless. They occasionally branch as in 

 the Gleditschia, thus showing that the pow r er of subdivision is 

 a vital quality inherent in the growing point itself. 



The spadix of the Arum, the receptacle of Nelumbium, all 

 the forms of placenta, and even some styles and stigmata are 

 modifications of the growing point of the bud, and conse- 

 quently are analogous to unhardened spines. 



Linnaeus called the bud Hybernaculum, because it serves 

 for the winter protection of the young and fender parts ; and 

 distinguished it into the Gemma, or leaf-bud of the stem, and 

 the Bulb, or leaf-bud of the root. 



The leaf-bud has been compared by Du Petit Thouars and 

 some other botanists to the embryo, and has even been deno- 

 minated a fixed embryo. This comparison must not, however, 

 be understood to indicate any identity between these two 

 parts in structure, but merely an analogous function, both 

 being formed for the purpose of reproduction ; in origin and 

 structure they are entirely different. The leaf-bud consists 

 of both vascular and cellular tissue, the embryo of cellular 

 tissue only : the leaf-bud is produced without fertilisation, to 

 the embryo this is essential : finally, the leaf-bud perpetuates 



