212 



BOTANY OF THE LIVING PLANT 



On the other hand, Hydrophytes, which grow in wet situations or 



|]y S ubn I, are independent of the risks of water-supply. 



|cav< • often finely divided, giving a large proportion of 



sur! to bulk, as in the water Buttercups. They are mostly 



rennials. The Water-Lily (Nymphaea) will serve as a good example. 

 Its thick stock is rooted in the mud and bears floating leaves, with broad 

 but thin lamina and smooth surface. Stomata are absent from the 

 submerged parts, but are present on the exposed upper surface 

 The shoot has little mechanical or woody tissue, but 

 contains large air-spaces which give buoyancy. The air-spaces 

 also allow of the storage of carbon dioxide or oxygen, and of gaseous 

 diffusion from point to point within the plant. The texture of such 

 plants is limp. Their parts dry up quickly in the air, owing to 

 deficient cuticular protection. These characters, which are com- 

 mon in the Hydrophytes, are in sharp contrast with those of xerophyte 

 types. 



The Mesophyte vegetation remains to be considered. Excepting in the 

 higher temperature, and the greater intensity of lighting of the latter, 

 the Temperate Zones and the Tropics are alike in presenting con- 

 ditions very favourable to growth, so long as extremes of season and 

 of water-supply are excluded. In the low lands of the temperate 

 zones and of the tropics, many areas exist where vegetation is easy. 

 Here, when supplied with seed produced by prolific methods, the soil 

 becomes covered with a dense investment of herbage or of woody 

 plants, in which the potential individuals are more numerous than the 

 ground can carry. Over-population is the character of the sward of 

 any field, as it is habitually of natural woods and forests. Two points 

 emerge from the contemplation of such native or natural growth. 

 One is that plants of very diverse outline and construction may thrive, 

 mixed indiscriminately together. The normal types of Monocoty- 

 ledons and of Dicotyledons seem to succeed equally well side by side. 

 This indicates that under such conditions there is little need for 

 specialised development. The second is that the overpopulation leads 

 to competition for space and light. Evidence of this is found in the 



mmonness of stunted plants, crowded out by the stronger. Any 

 area of densely overgrown ground in a lowland field or wood shows 

 in a convincing way how important access to sunlight really is. The 

 I ),an: ■ in fact, in a race for the light, and the tallest plants win. 



It is upon this fact that the most striking adaptive feature of the 

 Mesophytic and Tropic vegetation is based, viz. the Climbing Habit. 



