8 W. P. Wilson. 



species of maples and other trees riebest in saccharine substances. In 

 the birch he found 1,3^ sugar. The maples gave less. Schroeder i) found 

 for the highest per ct. of sugar in the sap of the birch 1,92, lowest 0,34 % . 

 With the maple the maximum reached 3,71 %^ minimum 1,15 ^ per ct. 



According to Unger the sap taken from a bleeding grape-vine on the 

 eighth of April gave a specific gravity of 1,0009 and showed 0,07^ of 

 sugar. Sap taken from another vine gave a sp. gr. of only 1,0001^ con- 

 taining almost no solid substance. This is sufficient to show the difference 

 existing in general between the sap and nectars of plants. 



It must be remembered also that the nectars often become very con- 

 centrated through the evaporalion of water. 



In many cases the sirup becomes so thick that sugar crystalizes out 2). 



Water Supply and its Relation to the Excretion of Nectar. 



The bleeding of plants in the spring and the appearance of water on 

 the leaves of may species, resulting from internal pressure arising in stem 

 and root, exists in very close relations to the evaporation from the leaves 

 and other rapidly growing parts. 



The bleeding decreases as the buds are developed, and generally 

 ceases entirely either before, or when the leaves are well unfolded and the 

 amount of evaporation becomes great. This excretion of water on the leaves 

 is seldom seen except in a very moist or altogether saturated atmosphere, 

 when the cells become fuUy turgescent and the water is finally forced out et 

 the stomata or water pores on the teeth or tips of the leaves. 



If the condition of the atmosphere becomes drier, the evaporation 

 from the leaves increases, and the excretion of the water from the water- 

 pores ceases just as soon as the amount of water taken up by the roots 

 falls below that passing off from the surface of the plant by evaporation. 



The excretion from the nectary bears no such direct relation to the water 

 supply of the plant or to evaporation from the leaves. The nectaries may 

 be active vt^hen the water supply falls far below the necessary amount for 

 the normal growth of the plant; even after the cells of stem and leaves loose 

 their accustomed turgescence and become wilted. The nectaries on the 

 leaves of Prunus laurocerasus, on a brauch which was placed on a table 

 without water in the dry air of an ordinary room, after having been kept 

 under a bell-jar until the nectaries were active, continued to excrete nectar 

 until the whole branch had lost more than one fourth of its weight. 



The excretion of water in the bleeding of plants and on the leaves is 

 the result of an internal pressure. The water flows out at the wound or 



1) ScHROEDER, Die landwirthschaftlichen Versuchsstationen. Band XIV. 1871. p. 168. 



2) KURR, Untersuchungen über die Bedeutung der Nectarien in den Blumen. 1832. 

 p. 108. 



