262 OKGANOGRAPHT. 



form of a stalk called the caudicle (Jig. 550, c), which adheres 

 commonly at the period of dehiscence to one or two little glan- 

 dular masses called retinacula (figs. 552, a, and 550, r), Avhicli 

 are placed on the upper surface of a little projection called the 

 rostellum situated at the base of the anther. 



We nmst now return to the more particular description of the 

 pollen-grain or cell. We shall treat of it under four heads, 

 viz.: - 1. Its Wall or Coats; 2. Its Contents; 3. Its Forai 

 and Size; and 4. Its Dehiscence. 



1. Wall or Coats of the Pollen-cell or grain. — When perfectly 

 ripe the wall of the pollen-grain generally consists of two 

 membranes; an internal or intine, and an external or extine. 

 In rare cases the outer coat appears to consist of two, or even 

 three layers; while in Zostera, ZannichelUa, and some other sub- 

 merged aquatic plants, there is but one membrane, which is of a 

 similar nature to the intine. 



The intine is the first formed layer, and appears to be of the 

 same nature and appearance in all pollen-grains. It is usually 

 smooth, very delicate, and transparent. It is generally applied 

 so as to form a complete lining to the extine, except perhaps 

 in those cases where the latter presents various processes, as in 

 (Enothera, when Henfrey believes that tlie intine does not ex- 

 tend into them in the mature pollen. 



The extine is a hard thick resisting layer forming a kind of 

 cuticle over the intine. While the intine always presents a 

 similar appearance in the pollen of different plants, the extine 

 is liable to great variation, thus it is some- 

 Fig. 553. times smooth ; at other times marked with 

 little granular processes (fig. 553), or spiny 

 protuberances, or reticulations, &c. The na- 

 ^^^^ ^^ these markings as already noticed, 

 3=''^ :•/]:.• }^X is always the same for any particular species 

 *^^* '"*'*'""" of plant, but varies much in different species ; 

 the mode in which they originate is at pre- 

 sent unknown, Tlie outer coat is generally 

 covered by a viscid or oily secretion, which 

 Fig. 553. Pollen of jg gypposcd by some, to bc dcrivcd from matter 



Hollyhock (AC- '.'. ^ "^ ^, i ... ^.., ,. i, 



thcca rosea). remammg trom the solution or the parent cells. 



The colour of jwllen-grains also resides in the 

 extine. In by far the majority of cases the pollen-cells are 

 yellow, but varioiis other colours are found ; thus tliey are red 

 in tlic Ver/juscum, blue in some s])ccies of Epilobium, black in 

 the Tulip, rarely green, and occasionally of a whitish tint. 



Besides the various markings just described as existing on 

 the extine, we find also, cither pores (fig. 556), or slits (figs. 

 554 and 555), or both together, and which vary in number and 

 an-angement in different plants. At the spots where these slits 

 or pores arc found, it is generally considered that the extine is 



