THE MATERIAL INCOME OF PLANTS 317 



Aerial permeable regions. — Land plants possess also certain per- 

 meable regions on the aerial parts of the shoot. Small plants that grow 

 in wet places, where the air is very moist or nearly saturated, might 

 safely have all aerhl parts permeable, because evaporation is slow and 

 the distance from root to aerial surface short. Moreover, spray or rain 

 falling on such parts may enter there, as well as soil water by the roots. 

 But larger plants could not exist in ordinary dry air were their permeable 

 aerial surfaces freely exposed; for if accessible to rain, the evaporation 

 would be dangerously great. So far as protection is concerned, large 

 plants with aerial shoots might thrive (1) if they were completely water- 

 proofed, thus checking all evaporation, or (2) if their damp surfaces were 

 shielded by drier partial coverings, thus reducing evaporation and 

 necessarily excluding water. 



Waterproofing vs. salts. — There seems to be no a priori reason re- 

 lated to the necessary supply of water and salts why the first of these 

 alternatives should not have appeared in land plants. Structurally, it 

 would be quite possible to waterproof the aerial parts completely, since 

 plants do check water loss by such means in certain places. In such a 

 case, enough water for other purposes might undoubtedly enter, since 

 enough to supply the great evaporation now enters by the roots alone. 

 But, it is objected, this would have prevented the intake of sufficient 

 salts. As to that, it is not probable that stopping evaporation, and 

 therefore the large inflow of water at the roots, would interfere with the 

 supply of salts. This is rendered probable, because diffusion of solutes 

 is independent of the movement of water; and to assume, as this objec- 

 tion does, that the solutes are carried along by the entering water which 

 replaces that evaporated, contravenes all that is known about osmotic 

 movement. Further, it is supported by the observation that in the rain 

 forests of Ceylon (and doubtless elsewhere) there are regions of luxuri- 

 ant vegetation where for months at a time the rain ceases only to be 

 replaced by a mist. In such conditions evaporation is almost impos- 

 sible. It cannot, therefore, be necessary to the adequate supply of 

 solutes from the soil. It is difficult or impossible to create such con- 

 ditions experimentally; and ordinary plants, accustomed to evapo- 

 ration, are so upset by being grown in a saturated atmosphere that 

 most culture experiments to ascertain the role of evaporation have 

 failed. , The few that have resulted in healthy development indicate 

 also that evaporation is not necessary, so far as a supply of salts is 

 concerned. 



