OBSEBVATIONS UPON THE POLLEN TUBE. 201 



form of small ingrowths from the wall of the tube. A 

 pair of these may be formed opposite each other and 

 growing together form a complete partition but as a 

 general rule the septum is formed as an ingrowth from 

 one side of the tube only. The ingrowths swell up, 

 commonly becoming tuberculated and may form irregular 

 amorphous masses blocking up the cavity of the pollen 

 tube. They are formed by the localised imbibition of 

 water causing localized swellings and the fact that these 

 are always found internally and never externally shews 

 that the inner part of the wall of the pollen tube is softer 

 less resistant and more capable of excessive imbibition 

 and of undergoing a partial mucilaginous modification 

 than is the outer layer. 



Exactly similar partitions though neither so numerous 

 nor distinct as in Narcissus, were found in normal cultures, 

 of one or more day's growth, of all the pollen grains 

 examined, viz., Cheiranthus, Vaccinium, Tulipa, Lathyrus. 

 They are therefore of very general occurrence and though 

 they are to be regarded as pathological growths are never- 

 theless in same cases apparently of importance. Thus in 

 an old tube of some length in which the basal part and 

 the cavity of the pollen grain itself are alike devoid of 

 protoplasmic contents it is evidently an advantage that 

 this portion should be, as it often happens in Narcissus, 

 cut off from the living apical portion of the tube and hence 

 the maintenance of turgidity necessary for the elongation 

 and growth of the pollen tube restricted to that portion 

 of the tube in which the cell-wall is still bounded internally 

 by a lining film of protoplasm. On the other hand the 

 formation of a septum across the middle of a tube cutting 

 off a portion of the protoplasmic contents cannot but be 

 disadvantageous and this is especially the case when, as 

 sometimes though rarely happens the part of the proto- 



