52 PRACTICAL BOTANY 



ARBOREOUS TYPE 



I. Note the following external characters of a twig of Elm 

 (Ulmus campestris) of the current year. It is cylindrical, 

 hirsute, green or brown according to age, the latter colour 

 being due to the formation of a superficial layer of cork. Small 

 brown excrescences are scattered over its surface ; these are 

 lenticeis. The arrangement of leaves is alternate and bilateral, 

 phyllotaxis ^, branching axillary : at the base of each leaf, note 

 on either side a small scar, where the stipule was inserted : the 

 stipules usually fall away soon after the buds open in spring, 

 but they may sometimes be found persistent for a time on 

 strong shoots. 



II. Cut thin transverse sections of a twig of the current year ; 

 mount in glycerine, and examine with a low power. Other 

 sections may, for comparison, be treated with chlor-zinc-iodine, 

 others again with aniline sulphate. 



Observe the general arrangement of tissues in concentric 

 layers, which will be found to succeed one another in the fol- 

 lowing order, starting from the outside : 



1. Epidermis : a single layer of small cells : many of them 

 have grown out, as conical hairs, perpendicular to the sur- 

 face. 



2. Cork : consisting of one or more layers of square cells : it 

 will be more strongly developed in older twigs, while it is com- 

 pletely absent in very young twigs : its development is studied 

 below, pp. 54, 55. Here and there a lenticel may have been 

 cut through, in which case it will appear as a lateral extension 

 of the band of cork. 



3. Cortical tissue : parenchyma with chlorophyll, having 

 cellulose walls, and intercellular spaces : an outer band of the 

 cortex shows a collenchymatous development, similar to that in 

 Helianthus. The inner part of the cortex is thin-walled : here 



