ABSORPTION AND TRANSMISSION OF WATER 101 



the cell wall and cause a renewed outward flow of water, and 

 the renewal of the droplet. 



The recent work of Haupt on extra-floral nectaries adds 

 to our knowledge of gland action. This author finds that 

 the secretion of sugar in these nectaries begins only when 

 the gland has attained a certain stage of development, and 

 then only when transpiration is relatively slight. This 

 makes it appear as though the protoplasmic layers become 

 permeable to sugar at a certain phase in the series of devel- 

 opmental changes, providing there be at that time a great 

 accumulation of water in the cells. This last provision 

 means that the cell walls and plasmic membranes are strongly 

 stretched. After secretion has begun an increase in humid- 

 ity causes an increase in the excretion of water from the gland, 

 but that of sugar remains constant. Hence it may be con- 

 cluded that the humidity, i. e., the amount of water evapo- 

 rated, has no direct effect upon the permeability of the 

 protoplast to sugar. For the beginning of the secretion, 

 Haupt finds that a certain minimum temperature is neces- 

 sary. The temperature very probably affects the physical 

 nature of the protoplasmic layer. That the red-yellow por- 

 tion of the solar spectrum influences the activity of these 

 glands in a profound manner has been already mentioned. 1 



It is important to note in this connection that, while pro- 

 toplasm is generally to be regarded as semi-permeable 

 toward many solutes, yet there is evidence from almost all 

 parts of the plant kingdom that it is often permeable to such 

 substances as sugar, acids, inorganic salts, etc. The pene- 

 tration of these substances is usually a slow process, how- 

 ever. Now the variation in the permeability of different 

 protoplasts, even of the same plant, may be taken as evi- 

 dence that slight differences in the protoplasm may cause 

 rather great differences in permeability. This makes the 



i See p. 78. 



