428 REPRODUCTION. 



1111. Structure of pollen-grains. 1 The grains consist of sin- 

 gle cells having a firm membrane and heterogeneous contents. 

 The membrane is rarely single (as in Zostera), being generally 

 composed of two coats, an outer, the extine (called exine by 

 Schacht), and an inner, the inline. The extine may be smooth, 

 but it is frequently beset with protuberances of some kind, points, 

 prickles, or other sculpturings, which may be characteristic of 

 genera or even larger groups. It is also provided generally 

 with one or more partial or complete perforations, which are of 

 course full}' closed by the intine which is pressed up against 

 them. The number of these perforations is constant in certain 

 groups of plants : for instance, one in most monocotyledonous 

 plants ; two in Ficus, Justicia, Beloperone ; three in Onagraceae, 

 Geraniaceae, Composite ; four to six in Impatiens, Ulmus, and 

 Alnus ; many in Nyctaginaeese, Convolvulacese, Malvaceae, and 

 some Caryophyllaceae. Under the action of concentrated sul- 

 phuric acid the intine is destroyed, while the extine generallj r 

 remains unchanged except in color. 2 



When the pollen of Thunbergia is acted on by strong sulphuric 

 acid, the destruction of the intine permits the extine to uncoil 

 as a band. In no case did Schacht detect any perforation of 

 the intine. 



1112. The contents of a pollen-grain are (1) protoplasmic mat- 

 ter ; (2) granular food materials, such as starch, oil, and, ac- 

 cording to Schacht, inulin ; (3) dissolved food matters, sugar 

 and dextrin. These heterogeneous contents form what was 

 formerly called the fovilla. 



In the granular protoplasmic matter of pollen-grains it is pos- 

 sible to demonstrate the existence of a nucleus, and in some 

 cases two nuclei can be made out distinctly. It is considered 

 well established 3 that the single nucleus which exists in the 

 simple grain at the period of its separation from the mother-cell 

 divides in most cases into two nuclei of unequal size. The larger 

 of the two fragmental nuclei remains with no change ; while the 

 smaller may become partitioned off from the rest of the cell either 

 by a true cell-wall or by a peripheral film of protoplasm, and may 

 later divide and form a group of two or four minute cells. 



1 These details are summarized chiefly from Schacht's exhaustive treatise 

 on the subject in Pringsheim's Jahrbucher, ii., 1860, p. 109. 



2 In some cases a double membrane can be shown in the extine, for instance 

 (Enothera, where the extine separates into a true extine and an intextine. 



3 Strasburger : Ueber Befruchtung und Zelltheilung, 1878. See also 

 Quarterly Journal of Microscopical Science, 1880, p. 19. 



