142 PRACTICAL BOTANY 



Saxifragacece (e.g. Saxifraga crustata) in which the structure of 

 the marginal gland is extremely well seen, while the accretions of 

 chalk deposited by the evaporating water are easily recognized 

 with the naked eye. S. oppositifolia or S. unibrosa (London 

 Pride) will afford excellent material for the study of these glands. 

 Compare also various Crassulacece. 



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

 solution with evolution of bubbles of C0 2 . 



IV. Extra-floral nectaries, Examine the back of the leaf 

 of the Cherry Laurel, and near the base of the lamina, on either 

 side of the midrib, will be seen circular areas, of variable number, 

 and with a smooth surface and opaque appearance : these are 

 glands, which produce a sugary secretion. 



Cut transverse sections so as to traverse one or more of the 

 glands : mount some in glycerine, others in chlor-zinc-iodine : 

 observe the transition from the ordinary epidermis of the lower 

 surface with the thick outer wall, to the glandular epithelium 

 consisting of cells which are narrow and deep, with plentiful 

 granular protoplasm and nucleus ; the lateral walls are thin ; the 

 external wall is rather thicker and covered by a continuous 

 cuticle (as shown on treatment with chlor-zinc-iodine), which may 

 however be separated from the wall as a thin film, and be rup- 

 tured. Note also a band of cells with peculiarly thickened walls 

 surrounding the margin of the glandular spot. 



Below the epithelium is a mass of parenchyma without inter- 

 cellular spaces : the walls are slightly thickened and pitted, and 

 the cells have plentiful protoplasm : this tissue merges gradually 

 into the ordinary mesophyll. Below the glands, and closely 

 connected with them, vascular bundles will be seen. 



Sections may also be made through similar glands on the leaves 

 of other species of Prunus, or of Viburnum Opulus : the dark 

 spots on the under surface of the stipules of Vicia show a different 

 structure, the epidermis there giving rise to numerous capitate 

 hairs : a sugary secretion is exuded in some species. 



Sections may also be made of the pocket-shaped glands on the 

 involucre of Poinsettia, previously hardened in alcohol. Cut 

 them longitudinally, and mount in glycerine ; or, better, stain with 

 Kleinenberg's hsematoxylin, and mount in Canada balsam. 



