452 REPRODUCTION. 



1160. From the nectar-glands of proper floral organs the secre- 

 tion of nectar is generally copious and is prone to collect in 

 minute cavities such as shallow pits, or in conspicuous special 

 receptacles, the so-called nectaries. The morphology of these 

 organs has been sufficiently described in Volume I., Chapter VI. 



1161. The specific gravity of nectar is very variable. The 

 following figures are from Unger's l determinations : 



Agave Americana 1.05 



" geminiflora 1.09 



" lurida 1.20 



If it is assumed that the solid matter in nectar is wholly sugar, 

 these figures would correspond respectively to the following 

 amounts of cane-sugar ; namely, 10, 18, and 41.66 per cent. 2 



1162. The period of most copious secretion of the nectar usually 

 coincides with the maturity of the anthers or of the stigma, but 

 in some cases the nectar is prepared in considerable quantity 

 before the flower opens. 3 



1163. The secretion of nectar can be arrested, as Wilson has 

 shown, by carefully washing the secreting surface with a jet of 

 water and then drying it with filter-paper. Nectaries which 

 have been thus made inactive through removal of the nectar can 

 be again brought into activity by adding to the surface a little 

 strong sjTup. 



1164. The secretion from nectar-glands is not dependent upon 

 the pressure exerted by contiguous cells. When the flow of the 

 nectar from a nectar-secreting surface has been arrested in the 

 manner described above, a pressure of even 40 centimetres of 

 mercury upon the stem is insufficient to produce any effect ; but 

 the activity of the surface is at once resumed when a little syrup 

 is placed upon it. 



The secretion of nectar can proceed even when the tissues are 

 not turgescent. 4 



1165. The colors of flowers depend, as indicated in 477, 

 upon the existence in the cells of minute granules or of colored 

 sap. The shades may be modified to some extent by accidents 



1 Sitzungsberiohte, Berlin Akademie, xxv., 1857, p. 446. 



2 Wilson : in Untersuclmngen aus dem bot. Inst., Tubingen, 188], p. 7. 



3 Bonnier : Les nectaires, Ann. des Sc. nat., ser. 6, tome viii., 1879, p. 5. 



4 For details see an important memoir by Wilson in Untersuclmngen aus 

 dem bot. Inst., Tubingen, 1881, i. p. 1; also an excellent paper by Trelease, 

 " Nectar and its Uses " (in Report on Cotton Insects, U. S. Dept. of Agricul- 

 ture, 1879). which contains a comprehensive bibliography. 



