igo PLANT ORGANS OR PARTS OF PLANTS. 



indigenous to many European countries, and is exten- 

 sively used in gardens. 



Description. — The leaf is simple and entire and wilts 

 very rapidly by reason of its open structure. Anatomi- 

 cally it is bifacial, the palisade tissues being found on 

 the upper side only. The epidermis of both sides is 

 similar, and the stomata are distributed on both sides. 

 The epidermis cells are irregular in shape, with wavy 

 outlines save over the main veins, where they are" some- 

 what elongated and sharp pointed. Hairs are present on 

 both sides. 



Histology. — The following structures are to be distin- 

 guished: Parenchyma, crystals, epidermis cells, hairs, and 

 fibro-vascular elements. 



The parenchyma is thin-walled, simple, and usually 

 rich in chlorophyll, often brownish in general color. 

 Palisade cells from the upper side only are in single rows, 

 rarely double, and these usually are connected at their 

 lower ends to the funnel-shaped cells of the mesophyll. 



In the irregular mesophyll cells numerous crystals of 

 calcium oxalate are to be found. These are very various 

 — column-shaped, dice-shaped, cuboidal, and octahedral 

 forms being found. Sometimes twin crystals are se^en. 



The hairs are very characteristic: both simple and 

 glandular types abound. Most of them are multicellular. 

 The simple hairs end in straight non-secreting points, 

 while others have many-celled heads which contain 

 resin-like secretions. The simple hairs vary greatly in 

 length and diameter. In length they often measure from 

 I GO to 400 microns, and often average bet wen 20 and 50 

 microns at the base. The wall is usually smooth. The 

 glandular hairs may be larger even than the simple ones. 



Stomata are frequent, being found on both surfaces. 

 They average about 40 microns in their longest diameter 

 and about 30 in breadth; the **neben-zellen" average 

 three to four, though there may be at times as many as six. 



