of the Structure of the Pollen Granule. 99 



sent a smooth membrane, an opposite result is almost always 

 encountered when one examines them when fresh. The mem- 

 brane of the bands is then so delicate that it is torn either 

 merely by the extension which the grain of pollen undergoes 

 in water, or by the separation of the external membrane in 

 such a manner that the bands appear to be true solutions of 

 the continuity of the external membrane, in which cases the 

 internal membrane is denuded by the swelling of the grain in 

 water. It was not long after this observation that the exa- 

 mination of pollen a long time dry set right my ideas upon 

 this point ; in fine, I have always been able to observe the 

 external membrane in the folds, in pollen in a dry state. It 

 appears then, that the external membrane acquires by drying 

 a very great hardness, while in the fresh pollen the concealed 

 part of this membrane in the folds appears to possess a con- 

 sistence rather gelatinous than membranous, from which it 

 follows, that not being able to bear any great extension, it 

 tears and presents itself in this state to the observer/' In an- 

 other place Mohl remarks, " but the portion which was con- 

 cealed in the fold always presents a structure different from 

 the rest of the membrane, although it is an immediate con- 

 tinuation of it." 



My own observations do not permit me for a moment to 

 doubt that these bands are formed, as already mentioned, by 

 the protrusion of the internal membrane through fissures in 

 the external, which are true and natural solutions of continuity 

 in that membrane. This view of their nature is supported by 

 several facts, which, when taken together, may safely be con- 

 sidered as conclusive: 1st, these bands differ from the exter- 

 nal membrane in being of a lighter colour ; 2nd, they are se- 

 parated from that membrane by well-marked and raised lines 

 of demarcation ; 3rd, in cases where the external membrane 

 exhibits a cellular formation, this structure cannot be traced 

 on to the bands, although Mohl asserts that in some instances 

 it may be thus traced : 4th, the polleu tubes are a growth or 

 direct continuation of the bands ; 5th, the bands are in some 

 cases covered by a valvular piece of membrane, which is really 

 a portion of the outer membrane, and when this is removed 

 the bands themselves become visible, presenting their usual 

 appearance ; 6th, the existence of fissures in the external mem- 

 brane may be demonstrated. In the dry pollen a fissure lies 

 concealed at the bottom of each of the furrows, which gene- 

 rally run lengthways from one extremity of the granule to the 

 other. In this state its edges are inverted and in contact, so 

 as to prevent the egress of the pollen tubes ; but as soon as 

 the pollen comes to be immersed in water or the stigmatic se- 

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