12 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



moist and cloudy ; its areas tend to be well vegetated, often with 

 broad-leafed forests or verdant pastures. The continental extreme, 

 on the other hand, is usually found far inland from the ocean and 

 tends to have low relative humidity and precipitation, though 

 exhibiting wide seasonal and daily fluctuations especially of tempera- 

 ture. The summer here is commonly sunny and warm but dry, 

 the winter being relatively cold, so that vegetation tends to be 

 limited, often consisting of drought-resistant Grasses, Heaths, or 

 desert plants. 



The Ideal Plant 



At this point will be given a brief account of the structure and 

 adaptation of a multicellular ' higher ' plant, such as a member of 

 the Angiosperms which top the ' evolutionary tree ' and are dealt 

 with at the end of the next chapter. Such flowering plants make 

 up most of the bulk of modern vegetation, give us very many of 

 our foods and other necessities of life, and consequently loom largest 

 in our plant geographical and allied studies. 



Our ideal plant, as we may thus conceive it, will consist of (i) 

 roots for anchoring in the ground and absorption from it of water 

 and soluble nutrients, (2) a stem to hold the leaves and reproductive 

 parts aloft, (3) green leaves to manufacture food substances in the 

 light, and (4) flowers to produce seeds and so eflFect reproduction. 

 Such features are too familiar to require illustration. 



Each main portion of a higher plant is composed of ' cells ', which 

 are minute and often box-like structural units that are variously 

 adapted to cover different needs. Cells of one kind are commonly 

 aggregated together to form ' tissues ' of particular form and function. 

 Thus some cells are for conduction — particularly of water and 

 dissolved salts upwards from the roots and of elaborated materials 

 downwards from the leaves — and are consequently elongated and 

 often pipe-like. Other cells have greatly thickened walls and give 

 tensile strength to roots and rigidity to aerial parts of the plant — 

 especially in the latter instance when aggregates of them are situated 

 near the periphery, as they commonly are in stems. Many cells on 

 the other hand remain thin-walled and serve the purpose of aeration, 

 food-storage, or mere ' packing ', while some may perform more 

 than one function either concurrently or consecutively. All kinds 

 of cells are produced from undiff"erentiated thin-walled * meriste- 

 matic ' ones which divide actively, for example in the growing-points 



