254 OKGANOGEAPHY. 



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

 wall of the pollen-cell 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, Zannichellia, and some other submerged 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-cells. 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 

 Oenothera, when Henfrey believes that the intine does not extend 

 into them in the mature pollen. 



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



cuticle OA'er 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 sometimes 



Fig. 553. smooth, at others marked with little granular 



processes {fig. 28), or spiny protuberances {fig. 



V;.vi-V 5o3), or reticulations {fiig. 557). The nature of 



Wv^»^°*^AL tli^se markings is always the same for any par- 



"l^^f^ 'S'/i:'^^ ticular species of plant, but varies much in dif- 



^^^>.''^y^«^V ferent species ; the mode in which they originate 



^^"^H'v*'''^ is at present unknown. The extine is generally 



''^^H^'^ covered by a viscid or oily secretion, which is 



supposed by some to be derived from matter re- 



Fin. 553. Pollen of niaining from the solution of the parent cells. 



thcearosea). The colour of pollen-cells also resides in the 



extine. In by far the majority of cases the 



pollen-cells are yellow, but various other colours are also found ; 



thus they are red in species of Vcrbascum, blue in some species 



of Epilohium, black in the Tulip, rarely green, and occasionally 



of a whitish tint. 



Besides the various markings just described as existing on tlie 

 oxtine. we find also either pons {fig. oo6), or slits {figs. 554,/, 

 and 555,/), or both pores and slits, and which vary in number 

 and arrangement in different plants. At the spots where these 

 slits or pores are found, it is generally considered that the extine 

 is absent; but some botanists believe that the outer membrane 

 always exists, but that it is much thinner at these points than 

 elsewhere. In the greater number of Monocotyledons, there is 

 but one slit ; while three is a common number in Dicotyledons. 

 Sometimes there are six, rarely four, still more rarely two, and 

 in some cases we find twelve or more slits. These slits are gene- 

 rally straight {fig. 554, /), but in Mimulus moschatus they are 

 curved ; and other still more complex arrangements occasionally 

 occur. 



