Mr. Hassall on the Structure of the Pollen Granule. 569 



may serve equally as a means of distinguishing it from those 

 orders to which it is evidently more or less closely allied. 



The difference in the size of the pollen tubes of granules 

 which in every other particular resemble each other, might 

 of itself, were there no other cause, constitute a physical im- 

 pediment to the fertilization of the seeds of one species by the 

 pollen of another, as, on account of their magnitude, they 

 might be unable to make their way through the intercellular 

 passages of the style. 



From all that has now been said it will be observed, that 

 from an examination of the form and structure of the pollen 

 granule, a useful confirmation may be gathered of the correct- 

 ness of the principles upon which a natural system of classi- 

 fication is based. The propriety, likewise, of employing the 

 pollen as a character, with others, in the definition of natural 

 orders, will, I think, be at once acknowledged, as I am glad 

 to see that it is by Dr. Lindley, who in a few cases has so 

 employed it ; but in the instances in which he has thus availed 

 himself of it, the true structure of the pollen appears to have 

 been either imperfectly or erroneously defined. As all the 

 essential characters of the pollen granule of each family may 

 be comprised in a few words, no difficulty can arise in em- 

 ploying it in the definitions of orders, sections of orders, or 

 genera, on the score of its adding too much to their bulk ; but, 

 at the same time, I would observe, that great care is necessary 

 in determining what the real structure of the pollen granule 

 is, before venturing to make use of it ; and this is not to be 

 ascertained by a momentary examination of it on the field of 

 the microscope, but requires frequently much patient and 

 troublesome investigation. 



For the numerous and beautiful drawings which accompany 

 this communication, all of which have been carefully executed 

 from rough sketches of my own, made of the object while un- 

 der the microscope, I am indebted to the friendship of two 

 ladies. Miss Hunter and Miss Nolcken, who are ever ready 

 to lend their time and their talents to works of usefulness ; 

 and that not a little of either is requisite in undertaking the 

 drawing of so many illustrations, will be readily allowed. About 

 two-thirds of them were done by the latter lady, and the re- 

 mainder by the former. 



It now only remains for me to acknowledge the great ob- 

 ligation I am under to Mr. Smith of the Royal Botanic 

 Gardens * at Kew, for the privilege so readily granted of ob- 



• Under the active management of their present director, Sir William 

 Hooker, as well as that of Mr. Smith, the usefulness and efficiency of these 

 Gardens will doubtless be much increased. 



Ann. §• Mag. N. Hist. Vol. ix. Suppl. 2 P 



