68 PRACTICAL BOTANY. 



STEM 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 cork 

 (cf. infra, p. 70). Small brown excrescences are scattered 

 over its surface ; these are lenticels. The arrangement 

 of leaves is bilateral, phyllotaxis ^, branching axillary. 



II. Cut 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 

 Schulze's solution, others again with aniline sulphate 

 and sulphuric acid.] 



Observe the general arrangement, of tissues in con- 

 centric layers, which will be found to succeed one 

 another in the following 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 surface. 



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

 cells : it will be more strongly developed in older twigs, 

 while it is completely absent in very young twigs (for 

 development cf. infra). 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, 

 and cellulose walls, and intercellular spaces ; here and 

 there are large transparent cavities (mucilaginous 

 cells). 



