ESSENTIAL ORGANS. POLLEN. 203 



ternal, inline (intus, within). Fritzsche states that he has detected, in 

 some cases, other two coverings, which he calls intextine and exintine. 

 They occur between the extine and intine, and are probably formed by 

 foldings of these membranes. In some aquatics, as Zostera marina, 

 Zannichellia pedunculata, Naias minor, &c., only one covering exists, 

 and that is said to be the intine. The extine is a firm membrane, which 

 defines the figure of the pollen-grain, and gives colour to it. It is either 

 smooth or covered with numerous projections, granules, points, minute 

 hairs, or crested reticulations (fig. 356). The colour is generally 



yellow, and the surface is often covered with a viscid or oily matter. 

 The intine is uniform in different kinds of pollen, thin and transparent, 

 and possesses great power of extension. It is said to be the first 

 envelope formed, the other being subsequently deposited while en- 

 closed in the parent cell. 



423. Within these coverings a granular semifluid matter, called 

 fovilla, is contained, along with some oily particles, and occasionally 

 starch. The fovilla contains small sphe- 

 rical granules, sometimes the ^^^0 ^ 



an inch in diameter (fig. 357), andlarger 

 ellipsoidal or elongated corpuscles (fig. 

 358), which are said to exhibit move- 

 ments under the microscope similar to 

 those seen in some Infusoria, and in 

 some Algas, to be afterwards noticed. 

 These movements generally cease long 

 before maturation, except in Zostera marina and some other plants. 



424. Pollen-grains vary from 5 J 5 to 7 i g of an inch or less in dia- 



Fig. 354. Pollen-grain of Passiflora before bursting, o o o, Opercula or lids formed by the 

 extine, which open to allow the protrusion of the intine in the form of pollen-tubes. 



Fig. 355. Pollen-grain of Cucurbita Pepo, or Gourd, at the moment of its dehiscence or rup- 

 ture, o o, Opercula or lids separated from the extine by the protrusion of the pollen-tubes, 1 1. 



Fig. 356. Pollen-grain of Ipomoea, with a reticulated extine. 



Fig. 357. Pollen-grain of Amygdalus nana, the intine or internal membrane of which is pro- 

 truding at three pores under the form of as many ampullae or sacs, tt t. One of these is open at 

 the extremity, and from it is discharged the foviUa, /, composed of variously-sized granules. 



Fig. 358. Large granules of fovilla of Hibiscus palustris. 



