198 FORMATIONS AND GUILDS [Part II 



impossible are confined to the tropics. The rain-forests of the tropics are 

 always moist. This is much less true of rain-forests of the warmer 

 temperate zones and not at all true of the summer-forests of higher 

 latitudes, for the cold of winter there constitutes a period of physiological 

 drought, which, even with the heaviest atmospheric precipitation, is more 

 opposed to the supply of water than great dryness when united with heat. 

 Under heat and dryness transpiration is indeed much greater, but the 

 absorption of water is not hindered and the nightly dew is of direct 

 advantage to the superficial roots of the epiphytes, whereas under temperate 

 conditions there is no supply of water to be set against its loss by epiphytes, 

 for the frozen or at any rate very cold exposed roots transpire, but absorb 

 nothing. 



Corresponding to these conditions of life, the vast majority of epiphytes 

 belong to tropical rain-forests. Only there do they luxuriantly cover 

 stems, branches, and frequently even the leaves of trees, and often them- 

 selves attain the dimensions of trees. In districts with markedly dry 

 seasons, and on the isolated trees of savannahs, epiphytes are either 

 completely wanting, or rare and represented by relatively few forms. Such 

 forms as are found are emigrants from the rain-forests, and their presence is 

 always a sign that the dry season is not long, or, as in the monsoon-forests, 

 is accompanied by copious dew. 



The origin of the guild of epiphytes in tropical forests may have come 

 about in the following way. Many terrestrial plants living in the forest are 

 able to settle and grow on rough fissured stems, in the forks of boughs, and 

 on other spots where humus collects. This happens in the tropics in the 

 case of many Solanaceae, Melastomaceae, and ferns. From such accidental 

 epiphytes true epiphytes were derived, since many of these plants owed 

 their existence to this faculty, which secured for them a safe retreat outside 

 the seat of conflict. The competition on the trees was limited to few 

 species, because the faculty of existing as an epiphyte demands certain 

 definite and by no means common characters. Obviously, for instance, 

 only such plants germinate on trees as are provided with seeds capable of 

 dispersal not only in a horizontal, but also in a vertical direction, and the 

 latter demands adaptations to arboreal animals and to the wind. Moreover, 

 the seeds must be very small, so that they can enter narrow crevices, and 

 in the case of dispersal by the wind they must be extremely light, because 

 vertical wind-currents are weak in the forest. The seeds of epiphytes 

 actually fulfil all these conditions ; they are always small, and either 

 surrounded by succulent envelopes, as in Aroideae, many Bromeliaceae, 

 Rubiaceae, Melastomaceae, Ficus, Cactaceae, and Gesneraceae, or they are 

 extremely light, even like powder, as for instance the spores of ferns, the 

 seeds of orchids, or they are provided, in spite of their very small dimensions, 

 with a most suitable parachute, as in Rhododendron, many Bromeliaceae, 



