24 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



non-developement of the axis takes place during the original 

 formation of the embryo. 



193. There is a constant tendency in opposite or verticillate 

 leaves to become alternate. 



194. This law applies equally to the arrangement of all 

 parts that are modifications of leaves. 



195. In leaves the developement takes place at their point 

 of junction with the stem ; and consequently the tissue at the 

 point of a leaf is the oldest. 



196. A leaf consists of a petiole or stalk, a lamina or blade, 

 and a pair of stipules. 



197. The PETIOLE is the channel through which the vessels 

 of the leaf are connected with those of the stem ; it is formed 

 of one or more bundles of spiral vessels and woody tissue, 

 enclosed in a cellular integument. 



198. The spiral vessels of the leaf of Exogenous plants de- 

 rive their origin from the medullary sheath ; those of En- 

 dogenous plants from the bundles of fibro-vascular tissue. 



199. The cellular integument of the petiole is a continuation 

 of that of the bark. 



200. When the petiole is leafy and the lamina is abortive, 

 it is called a phyttodium". 



201 . When the petiole becomes dilated and hollowed out at 

 its upper end, the lamina being articulated with and closing 



