160 



THE VEGETABLE KINGDOM. 



12. ALISMACE.E (Water Plantain), 



The two chief members of this 

 order are the Alisma Plantago 

 or Water Plantain, which grows 

 in ditches, having its flowers in 

 the form of panicles, and the 

 Sagittaria sagittaefolia or Arrow- 

 head. 



Waler Plantain. 



The third great natural family of plants are the Exogenae. 

 They comprise all the trees and shrubs of the temperate and 

 colder regions, together with many of the flowering plants. 

 They are characterised by certain peculiarities which can be 

 at once recognised, such as the twisted and branched form 

 of the stem, the possession of bark, leaves having the veins 

 covering them running in all directions and forming a net- 

 work, and the seeds containing two cotyledons ; the wood, 

 moreover, is deposited in rings (figs. 16, 17, 18, 19), one of 

 which is formed every year, by the new wood being pro- 

 duced on the outside of the old, and between it and the 

 bark. This deposition takes place as follows : after the rains 

 of winter and early spring have well saturated the earth 

 with moisture, and the warmth of spring has begun to pene- 

 trate to the roots of the plants, a development of the points 

 of each fibrile or radicle takes place, forming new spongioles ; 

 these, being formed of new porous cellular tissue, begin to 

 absorb (by endosmose) the moisture of the* earth, which 

 entering at all these thousands of minute spongioles at once, 

 collects and rises in the vessels of the trunk and branches, 



