490 Mathewson : Pollen-tube in Houstonia 



and so reaching the hair. In a similar case the tube grew out 

 toward the adjacent papilla and pursued its way thereon instead of 

 on the one with which the grain was in contact. Strasburger 

 figures cases in Agrostemma and Malva in which the tube enters 

 the papillary cell and pursues its way within, but I have found no 

 evidence that this occurs in the form I have studied. Growth on 

 the papilla is entirely superficial. The tube very often makes one, 

 or even two full turns around a papilla before it reaches the base, 

 a condition precisely like that described by Strasburger in the case 



of one of the grasses, Alopccurus pratensis. 



Arrived at the base of the papilla the tube begins its intercel- 

 lular course. The papillae merge at their bases with long, narrow 

 relatively thin-walled cells, the first kind of conductive tissue which 

 it meets in its course. There are no intercellular spaces whatever 

 in the tissues traversed by the tube. The width of the pollen- 

 tube varies from 2 to 4/*. It seems quite probable that the 

 method by which the tube first penetrates between the cells of the 

 epidermis of the stigma is the method by which it is able to force 

 its way throughout its entire course. It is well known that the 

 pollen-tube is capable of secreting a cellulose-dissolving enzyme, 

 and it seems that we should be safe in saying that by breaking 

 down the middle lamella between the cells with which it comes in 

 contact the slender tube of this species makes room for its growth. 

 The tube is doubtless nourished, in part at least, by the absorption 

 of the cellulose which has been converted into available food by 

 its enzymes. The width of the tube is not constant, usually being 

 somewhat greater in its younger portion than farther back, where 

 it becomes collapsed. The vascular elements of the style are in 

 two small bundles which are always farther from the middle of the 

 style than the pollen-tubes. Little or no distortion or other bad 

 effect results from the contact of the pollen-tube with the cells 

 along its course. If the tube meets a cell lying directly across its 

 path it simply passes around it. 



At the level of the roof of the ovary the pollen-tube encoun- 

 ters a tissue, the cells of which are more irregular and the walls 



thicker (figure 1). Some of the cells are quite short. The tube 

 goes on down through this stylar portion of the ovarian partition 

 as readily as through the part above. Anatomically there appears 



