98 Mr. H assail on MohVs views 



Mohl's explanation of the nature of these granules and of the 

 constitution of this membrane appears to me to be satisfac- 

 tory and philosophical ; but I am far from going to such an 

 extent as to suppose that the extine is never a simple organ, 

 analogous to a primitive vegetable cell ; on the contrary, I be- 

 lieve it to be most commonly simple. 



Mohl considers that the spines and papilliform eminences 

 which cover the external membrane of many forms of pollen 

 take their origin in a special development of one of the grains 

 of the external pollenical membrane, and that they constitute 

 a partial development of the granular membrane into the cel- 

 lular. This supposition of MohPs is decidedly erroneous. I 

 have already declared that most, if not all, kinds of spinous 

 pollen have a cellular external membrane, and that the spines 

 themselves owe their existence to an extraordinary develop- 

 ment of the outer surface of each cell, in the same way as the 

 hairs of the epidermis of plants are produced. Concerning 

 the nature of the papilliform projections, Mohl appears to me 

 to be far astray. They are not occasioned by any prolonga- 

 tion of the external membrane, which has nothing whatever 

 to do with their formation ; they are produced by the bulging 

 out of the intine when there are but two membranes, or of 

 the intextine when there are more than two, through either 

 fissures or apertures in the extine. 



It has been observed before, that on the immersion of those 

 forms of pollen which exhibit furrows upon their surface in 

 w^ater, or any other fluid of the same consistence, that the 

 plaits disappear, and that bands of a lighter colour than the 

 rest of the surface of the granule occupy the position of the 

 plaits, the appearance of these being accompanied by a re- 

 markable change in the shape of the granule in all those cases 

 where it is of an elongated form ; this change arising from the 

 approximation of the extremities of the granule, occasioned 

 by the imbibition of the surrounding fluid by endosmosis. 

 These furrows vary considerably in number, from one to up- 

 wards of twenty, a single furrow, for the most part, being cha- 

 racteristic of a Monocotyledon and three of a Dicotyledon ; 

 and the bands which supply their place subsequently are in- 

 variably disposed in a radiating manner, are narrow at their 

 commencement near the centre of the figure, and widen 

 gradually as they approach the circumference. These light 

 bands Mohl regards as spaces of the extine, thinner than the 

 rest of the membrane, and consequently more transparent. 

 He thus speaks of them : — " In all cases where the bands are 

 dotted, there is no doubt that the external membrane forms a 

 vesicle completely closed ; but upon pollen whose bands pre- 



