430 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



at any period, though a new crop soon develops on them. In this, 

 as was already indicated on pp. 424-5, particular species tend to 

 have their own special times — for example, members of the genus 

 Hevea in South America commonly lose their leaves at the end of 

 the dry season, regularly, each year, just before flowering. 



2. Herbs, etc. Where the tree strata are not too dense and 

 sufficient light penetrates, there may be a fair development of green 

 ground-vegetation which, like the dominant trees, is independent of 

 external support. Such lower vegetation in moist situations tends to 

 be largely herbaceous. Ferns and Selaginellas being often prominent, 

 whereas on dry ridges it may consist largely of woody plants. In 

 other cases a shrub stratum (D), consisting mainly of tallish woody 

 plants, may be roughly distinguishable, with, below, a ground-layer 

 (E) of herbs and tree-seedlings up to 2 metres in height. The 

 shrub layer often includes some coarse herbs such as Scitamineae 

 (Bananas, Gingers, etc.) which may exceed 5 metres in height. But 

 in general, in spite of the prevailingly warm and humid conditions, 

 herbs and other lowly plants are little developed on the ground 

 owing to the lack of sufficient light. Thus in lowland rain forest 

 any luxuriant herbaceous ground-vegetation is found chiefly in 

 clearings and by streams and openings where illumination is above 

 the average for the level, while in the interior of the forest green 

 herbs — apart of course from epiphytes — are found chiefly as widely 

 scattered individuals or scarcely at all. On steep slopes, however, 

 more light tends to penetrate owing to the angle of the (lateral) 

 rays, and herbaceous vegetation is generally more abundant, though 

 still the number of herbaceous species is liable to be far smaller than 

 that of diflferent trees. Indeed, in contrast to the situation in 

 temperate regions, the herbaceous vegetation in tropical rain forests 

 is almost always far less various than the arborescent, and, with 

 relatively ' open ' conditions, is more apt to form ' families ' of single 

 species. The herbs belong to various (if usually few) systematic 

 groups but typically include members of the Madder family 

 (Rubiaceae) as well as some Grasses and members of the Sedge 

 family (Cyperaceae) in addition to Ferns and Selaginellas, and, 

 in the Amazonian region, Marantaceae and Melastomaceae. Their 

 foliage is usually thin, sometimes variegated, and very variable in 

 shape, in contrast to that of the dominant trees. 



3. Climbers. We now come to the first of the three groups of 

 plants that, although they are still green, are dependent on external 

 mechanical support and afford the main ' forest furnishings ' ; of 



