PLANT STRUCTURE 147 



Healthy plant growth cannot go on without correct 

 moisture conditions in the atmosphere surrounding plants. 

 We have seen that large amounts of water are absorbed by 

 the roots. This water serves as a medium to convey food 

 substances about the plant, and while it furnishes hydrogen 

 and oxygen for plant food, the large amounts taken in are 

 not needed. Surplus water, therefore, passes off from the 

 leaf through transpiration. Upon the proper transpira- 

 tion of moisture from the leaf surface of plants, depends in 

 a large measure the healthy development of the species. 

 This, too, calls for careful attention to ventilation in green- 

 houses. For example, transpiration is more rapid in light 

 than in darkness, because the stomata are open in the light 

 and they facilitate the escape of water from the leaf. 

 Transpiration is more rapid under dry atmospheric condi- 

 tions than under moist. It is, therefore, necessary for the 

 florist to moisten the walks frequently and to fill the air 

 with moisture several times during the day, especially in 

 bright, sunny weather. This prevents too great a trans- 

 piration of water, which would injure the plant. Some 

 plants cannot thrive in a dry atmosphere. Rapid trans- 

 piration kills them. Nature prepares such plants for a dry 

 atmosphere by giving them a thickened epidermis or by 

 diminishing the number of stomata, so that transpiration 

 is checked. Plants which make a rapid growth need to 

 transpire more than slow growing plants. Houses for 

 palms, ferns and other tropical plants should have a 

 moister atmosphere than houses filled with plants from 

 temperate sections, as roses and carnations, which are be- 

 ing forced into rapid development. 



112. Light conditions. In considering the correct 

 atmospheric conditions for healthy plant development, 

 reference was made to Kerner's work regarding nutrient 



