428 INTRODUCTION TO PLANT GEOGRAPHY [cHAP. 



(or more often co-dominant) above. Indeed one of the striking 

 characteristics of most tropical rain forests is the extremely mixed 

 dominance, so that a species commonly occurs only from one to 

 three times in an acre. Local consociations of single dominance 

 may, however, be developed (Fig. 142), though in real tropical rain 

 forests this appears to be rare. The leaves of the trees are com- 

 monly of medium size, having an area of 2,000-18,000 sq. mm. 

 They are usually entire and ' leathery ', and dark-green with glossy 

 surfaces. Thus they belong to the laurel or large-sclerophyll type, 

 being mostly oblong-lanceolate to elliptical in outline, often with 

 extended ' drip-tips '. However, the type of leaf varies considerably 

 with the stratum, drip-tips, for example, being scarcely ever found 

 on leaves of mature trees of the higher strata. In many tropical 

 rain forests, foliage extends almost continuously from the herbs on 

 the ground to the tops of the dominants, and although many large 

 trees are present, this foliage of one sort or another mostly hides 

 their trunks (Fig. 144). In other instances the canopy is exceed- 

 ingly dense and there is little development of undergrowth and 

 ground-covering, so that the trunks of the trees stand out in the 

 gloom as huge columns. Often they show ' plank ' buttresses as 

 in Fig. 145, where an intermediate amount of ground-vegetation 

 is visible. 



Although it is often contended that competition between the trees 

 finds expression chiefly in the struggle towards the light, actually 

 the different strata have each their own species. Normally these 

 individually reach their own particular level before attaining full 

 development and thereafter make no attempt to pass that level. 

 The arboreal species thus seem to fall into groups having a" particular 

 height-limit and degree of tolerance to shading by the next stratum 

 above — or, in the case of the topmost stratum, presumably demand- 

 ing full exposure — the struggle towards the light being mainly in 

 the immature stages. Nor does there appear to be any intense 

 struggle for root-room — a feature that has been confirmed by Dr. 

 R. E. Schultes {in litt.), with the rider that it is unexpected and 

 more ought to be made of it. 



It should be recalled finally that in these tropical rain forests, not 

 only flowering and fruiting but also the loss and replacement of 

 leaves can take place at any time of the year and in fact normally 

 does take place at all times (considering the vegetation as a whole). 

 Thus in many species the leaves appear to be renewed annually, 

 and individual trees devoid of leaves may be observed in the forest 



