SCAR OF LEAF 83 



cognised with the naked eye. S. oppositifolia or S. umbrosa 

 (London Pride) will afford excellent material for the study of 

 these glands. Compare also various Crassulacecs. 



Treat some of the accretions with acetic acid, and note their 

 solution with evolution of bubbles of CO 2 . 



Leaf -scars and Fall of the Leaf. 



On twigs of the Elm cut in winter, note the buds, both 

 terminal and lateral, and below each an oval scar which indi- 

 cates the surface of separation of a leaf when it fell in autumn : 

 the surface of the scar is brown, and the slightly projecting 

 dots upon it are the broken ends of the vascular bundles which 

 ran out from the stem into the petiole. 



Cut longitudinal sections so as to pass through a scar, 

 and select for observation one of those which has followed up 

 the course of one of the vascular bundles to the surface of the 

 scar : mount in glycerine, and observe below the scar the 

 tissues as above described for the stem (p. 60, &c.). At the 

 level of the scar the following structural points are to be 

 noted : 



1. The rough and irregular outer limit of the tissues, with 

 dried up remains of cells often projecting beyond the general 

 surface. 



2. The dark brown band of cork, without intercellular 

 spaces, which covers the scar, and protects the internal 

 tissues. 



3. The projecting end of the vascular bundle : mark especi- 

 ally how the corky formation is continuous into the tissues of 

 the bundle, evidently having there arisen from the division of 

 cells of the parenchyma of the bundle : also note that the 

 vessels of the xylem are laterally compressed by the adjoining 

 cells, and their broken ends are thus closed at the surface of 

 the scar. 



Sections should also be made from material taken in autumn 

 just at the period of the fall of the leaf, so as to see the 

 changes in the tissues at the base of the petiole which precede 

 the rupture. These may be particularly well seen in the 



G 2 



