294 PLANT LIFE. 



membrane, shaped like the propeller of a steamer ; and 

 as the weight is slung obliquely, they twist and revolve 

 so that they take a long time to fall and may be 

 carried to a great distance. 



The distribution of the Pines and the other Conifers 

 most nearly allied to them is very interesting and 



Fig. 37. — Leaf of Akaucaria (seen in transverse section). The strengthening 

 or mechanical system consists of the epidermis (i), the layer under the epidermis 

 (3), the endodermis (5), and the white strengthening sheath (7). Water enters 

 the leaf by the xylem (8), and is carried by special valved cells (6) to the green 

 cells (4), which have peculiar shelves on their walls appearing as points in the 

 figure ; the water not required escapes by the stoma (2), which is sunk below the 

 surface. Near the stoma is seen a resin canal, protective in function. The 

 phloem by which food-material is carried away is seen at (9). 



instructive. A general survey of the world, from the 

 Arctic to the Antarctic, shows that Siberia, Northern 

 Canada, and Northern Scandinavia, are all largely 

 covered by pine woods. There is a Coniferous belt in 

 both the Pyrenees and the Alps, above that of the 

 Oaks and other trees. Even the isolated mountain 

 strips of the Lebanon and Mount Atlas possess the 

 Cedrus Libani and Cedrus Atlantica. The Canaries 

 had their woods of Pinus Canariensis, and the small 

 mountain group of Milanji, away in the extreme centre 

 of Africa near Lake Nyassa, has its forests of Wid- 

 dringtonia Whytei. Indeed, everywhere on the earth's 

 surface we find particular kinds of Conifers associated 



