96 ALDRIDGE ON POLLEN. 



thetnum nervosum and strictum, Barleria longifolia^ and many 

 MalvacecB, it is only necessary to moisten tlie pollen with 

 concentrated sulphuric acid, and then to press between two 

 glass plates ; the whole Exine shows itself so, as plainly to be 

 recognised, and demonstrates in this situation not only all 

 its peculiarities of condition, which in no other way we can 

 find out, but presents the isolated exhibition of the membrane 

 and its coverino-s. These two constituents of the Exinehaye 

 many characters in common, which the author mentions, 

 previously to describing each individually. 



The action of concentrated sulphuric acid, shows that a 

 difference exists between the Exine and the Inline^ and the 

 general cellular tissue. By the long-continued action of the 

 acid, the texture of the external appendages of the Exine 

 becomes more obscure, but short maceration will render 

 their general structure more distinct, and those points are 

 best to select for observation, which can be most plainly dis- 

 tinguished when, after maceration in acid, they have been 

 for some time excluded from the air by covering with water. 

 The Exine frequently acquires by this treatment a fine pur- 

 ple-red colour, whereby the examination of its structure 

 becomes facilitated. The more concentrated the acid, the 

 deeper becomes the red ; and, the more completely it is ex- 

 cluded by the water from the air, the darker will it change 

 by degrees. Generally, the membrane and the external 

 appendages conduct themselves alike ; but cases occur when 

 the first reddens itself, and the latter acquires a yellowish- 

 brown colour, through the action of sulphuric acid. 



The specific differences of the Exine depend on the 

 nature of the external appendages spread over its surface. 

 These consist in grains, or prickles, or warts, or a celhdar 

 lacework ; and are most beautifully delineated in the plate 

 attached to the author's paper. 



The last chapter contains a classification of the various 

 kinds of pollen. The author arranges them according to 

 the number of membranes ; whether there be one, two, three, 

 or many present. But 1 have alreadv extended this analysis 



