82 CONTINUITY OF TISSUES [CH. VII 



A point which is of importance to us here, however, 

 is the following. Each of the structural systems above 

 referred to is continuous with its like in the stem, root 

 and other parts of the plant. The epidermis of the leaf 

 stretches over not only the whole surface of the lamina 

 and petiole, but is absolutely continuous as epidermis 

 over the shoot which bears the leaf. The vascular system 

 riot only pervades the whole leaf- area, but is continued 

 down the petiole and joined on to the similar vascular 

 system running throughout the shoot and root and into 

 every branch, bud, tendril, flower, &c., which it puts forth. 

 In like manner the soft mesophyll which fills up all the 

 intervals between the vascular system (venation) of the 

 leaf and its epidermis, is continuous through the petiole 

 with the same kind of intervening soft tissues between the 

 epidermis and the vascular system of the shoot, and, by 

 its continuity with this, is continuous with the correspond- 

 ing tissues in every root, branch, flower, &c., throughout 

 the plant. 



This continuity of the tissues of like kind throughout 

 the plant is a matter of the utmost importance. Any one 

 of the systems may undergo rupture and destruction sub- 

 sequently, in the older stems, roots, &c., but it is primarily 

 absolutely continuous throughout, and, similarly, it never 

 joins another system, though it may abut closely on to it ; 

 each system is sharply marked off from the other from 

 first to last. See Fig. 56 and pp. 102105 in Vol. I. 



After this introductory survey, we may now examine 

 each of the three systems somewhat more closely. 



