STRUCTURE OF THE PISTIL. 427 



1107. Three subjects must be briefly reviewed before enter- 

 ing upon the stud}' of the process itself; namely, the pistil, the 

 ovule, and the pollen-grain. For all details regarding particu- 

 lars of form and special morphological relations, pages 249-285 

 of Volume I., and Chapter IV. of the present volume may be 

 consulted. 



1108. The aiigiospermous pistil (see Fig. 196) consists of a 

 closed ovary containing the ovules, which is generally prolonged 

 into a slender organ known as the style. Either some portion of 

 the style, or, when this is wanting, some portion of the ovaiy, is 

 furnished with a peculiar secreting surface known as the stigma. 

 The manifold shapes of ovary, st3~le, and stigma have been suf- 

 ficiently described in Volume I., and the microscopic structure 

 of each has been examined in a general way in Part I. of the 

 present volume. From what was there said, it will be remem- 

 bered that the form and structure of pistil and stamens have 

 intimate relations to the transfer of pollen and its reception by 

 the stigma. 



1109. The stigmatic secretion. The surface from which this 

 exudes may exist as an expanse of considerable extent, or it 

 may have the form of single or double lines, or be reduced 

 even to a mere point. The extent of the stigmatic surface 

 bears a fixed relation to the number of ovules in the ovary. 



At a certain period in the development of the flower, the 

 stigma, which up to that time may have been apparently free 

 from moisture, becomes covered with a glutinous secretion of 

 a saccharine nature. At this period, known as that of ma- 

 turity, the stigma is from its stickiness likely to catch and 

 retain upon its surface any pollen which may fall thereon. 

 The secretion is generally slightly acid l in reaction, and is as 

 variable in the amount of sugar which it contains as ordinary 

 nectar. 



1110. The pollen-grains of angiosperms when set free from 

 the cells in which they are produced may become completely 

 isolated (simple grains), or they may remain firmly coherent in 

 clusters of four (Typha, Rhododendron, etc.), eight, sixteen, 

 thirty-two, or even, as in some species of Acacia, sixty-four 

 ("compound grains"). In many Orchidacese the grains are 

 more or less compactly fastened together into masses by a glu- 

 tinous matter forming pollinia, and much the same grouping 

 into masses occurs in Asclepiadaceae. 



1 Van Tieghem : Traite de Botanique, 1884, p. 850. 



