MODIFICATIONS OF FORM 179 



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 

 commonness 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 

 plants engage, 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 

 Mesophyte and Tropophyte vegetation is based, viz. the Climbing 

 Habit. 



The Climbing Habit. 



The biological advantage gained by the climbing habit is that the 

 plant which adopts it reaches the light with a minimum expenditure 

 upon its stem. A plant standing alone has to form a strong supporting 

 column. To do this requires a considerable expenditure of material 

 on tissues which are of little physiological use beyond giving mechanical 

 support. If then such support can be attained in some other way, 

 so much material will be gained. That the expenditure is really 

 saved by climbing plants is seen from their anatomy ; for their stems 

 show vessels relatively few and large, few other tissues of the wood, 

 and in herbaceous types, though cambium may be present, there is 

 an absence of tissue-masses formed by cambial thickening. There is, 

 however, a well-developed phloem, which in some cases is duplicated 

 on the side next the pith. The vascular strands thus constructed 

 contain little fibrous tissue, and are usually isolated one from another 

 by intervening tracts of soft parenchyma (compare Fig. 25, p. 41). 



