50 LEAVES. 



la examining powdered leaves it is only rarely that a portion 

 can be found presenting a transverse section. Fragments of 

 the epidermis almost always present their surface to the ob- 

 server, and glands are, therefore, usually seen in surface view. 

 In this case unicellular and bicellular glands appear ovoid or 

 rounded, but quadricellular and! octocellular glands exhibit a 

 circular outline, the circle being divided into as many com- 

 partments as there are cells ; in the centre the pedicel may 

 be seen. In each cell of the gland droplets of volatile oil may 

 often be observed. 



The leaves of labiate plants bear both quadricellular and octo- 

 cellular glands, in addition to which unicellular and bicellular 

 ones can fairly often be found. 



Crystals. 



The presence or absence of crystals, and, in the former case, 

 'their shape and nature, afford valuable evidence in establishing 

 the identity of a leaf. They generally consist of calcium oxalate 

 and are usually rather small. Whenever they exist in a leaf 

 they are always to be found in the powder, however fine it may 

 be. Some leaves (foxglove) are entirely free from crystals, but 

 most officinal leaves contain them distributed throughout the 

 mesophyll, the cortex of the midrib, and the bast of the midrib 

 .and veins. Their shape generally remains constant in leaves of 

 the same plant, but it may vary for different plants of the same 

 natural order. Thus, in belladonna leaves the calcium oxalate 

 usually assumes the form of sandy crystals, in henbane that of 

 small prisms, in stramonium that of cluster crystals. In senna 

 leaves both prisms and duster crystals are found. 

 Secretory Tissue. 



Perhaps no part of the anatomy of a plant is so constant in 

 the form that it assumes and in its localisation as the secretory 

 tissue. It may, therefore, afford the most valuable informacion, 

 and should be carefully studied. The following forms of such 

 tissue may be met with in leaves : Oil cells (Laurinece, 

 Piperacece) internal oil glands (Myrtacece, Rutacece), secretory 

 ducts (Umbelliferce), lacticiferous cells (Euphorbiacece) , and 

 laticiferous vessels (Campanulaeece). 



Mesophyll. 



The mesophyll is said to be homogeneous when all its cells 

 assume the same form, but heterogeneous when they assume dis- 

 tinctly different forms ; the latter is by far the more common, 

 most leaves exhibiting a palisade tissue well differentiated from 

 the spongy parenchyma. The palisade cells often adhere closely 

 to the upper epidermis, and are not completely separated by 

 powdering, hence in the powdered leaf they are generally seen 

 in surface view, and appear then rounded and of uniform size. 

 They may also be found isolated, or in groups, and presenting 

 their rectangular longitudinal section to the observer. The 

 regularity of their shape and the quantity of chlorophyll they 

 contain allow of their being generally easily recognised. 



