GROWTH OF PLANTS 137 



its surface. These two magnitudes are connected in a definite 

 relationship; viz., 1 per cent decrease in the vapor pressure 

 corresponds to an increase of approximately 13.5 atmospheres 

 in osmotic pressure. 



The connection between growth capacity and the degree .of 

 saturation of the surrounding atmosphere is most obviously 

 manifested by lower organisms, the cells of which when active 

 are not protected against water loss into the surrounding medium 

 and consequently must be in hydrostatic equilibrium with the 

 latter. According to the observations of Walter, many molds 

 begin to retard their growth at 98 per cent relative humidity of 

 the atmosphere, and at 95 per cent, growth is completely stopped. 

 Only the most resistant species can continue growth in a medium 

 where the humidity falls to 90 or 85 per cent relative humidity ; 

 this corresponding to an osmotic pressure of about 127 to 217 

 atmospheres. 



In higher plants, the cells of the growing point of the roots 

 do not have protective coverings and are entirely dependent 

 upon the hydrature of the surrounding medium. For this 

 reason, roots are capable of growing only in a sufficiently moist 

 soil, whose interstices contain air almost completely saturated 

 with w^ater vapor and where the osmotic pressure of the soil 

 solution does not exceed 10 to 15 atmospheres. In a dryer soil, 

 root growth of most plants is not possible, and only roots of some 

 of the desert plants that are provided with protective tissues are 

 capable o^ pushing through dry soil. 



Aerial parts of the plant almost always remain in a com- 

 paratively dry atmosphere, the relative humidity of which falls 

 at midday to 40 to 50 per cent and frequently considerably lower. 

 The growth of these parts is possible only for the reason that the 

 meristematic tissue of the growing points is protected from direct 

 contact with the dry atmosphere by the closely packed leaves of 

 the bud, while the lower zone of elongation is covered by an 

 epidermis with a cuticle impermeable to water. A similar 

 epidermis covers the leaves, and owing to its presence the inter- 

 cellular spaces adjacent to the living cells of the growing leaf 

 tissue possess an atmosphere of not less than 99 to 98 per cent 

 relative humidity. 



As every increase of the cell-sap concentration involves a 

 corresponding withdrawal of water from the protoplasm, a close 



