492 Mathewson : Pollen-tube in Houstonia 



isodiametric cells and the path of the tube in it may be straight or 

 subject to sudden variations. As the tube approaches the ovule 

 it usually grows toward the interior of the placenta, so that by the 

 time it reaches the region of the funicle, it may be six cell-rows 

 below the surface. The cells of the funicle constituting a scission 

 layer, are much smaller than the parenchymatous cells of the 

 placenta and have their long axes at right angles to the path of 

 the tube. The tube usually makes many turns in passing through 

 this tissue. It may even grow for a minute distance across the 

 sinus between the funicle and the ovule. 



The tube has now reached the tissue of the ovule, composed 

 of small thin-walled cells, and its course, as in Alchemilla (Mur- 

 beck), in the region of the egg is subject to sudden deviations. 

 The general direction, however, is toward the egg end of the em- 

 bryo-sac, which it finally reaches. The course of the pollen-tube, 

 it will be seen, is very closely analogous to that in Alchemilla 

 (Murbeck), in the Cannabinaceae (Zinger), in Cynomoriiim (Pirotta 

 and Longo), and in Ulmus (in some instances studied by Nawaschin). 



Summary and conclusions 



The cells with which the pollen-tube comes in contact influence 

 it only in a passive manner. 



The influence of the pollen-tube upon the cells with which it 

 comes in contact is very slight. 



The course of the tube seems to support the hypothesis that 

 has been advanced that a chemotactic stimulus originating in the 

 egg-apparatus, or the egg itself, is the chief directive influence. 



The endotropic phenomenon here set forth adds another link 

 in the chain of evidence that the intercellular mode of growth is 

 purely a physiological phenomenon, and not a character inherited 

 from more primitive forms. i 



Literature 



Lloyd, F. E. Comparative embryology of the Rubiaceae. Mem. 



Torrey Club 8 : 1-26. 1899; 27-112. 1902. 

 The pollen-tube in the Cucurbitaceae and Rubiaceae. Torreya 



4: 86-91. 1904 



Houstonia 



Torreya 5: 83-85. 1905 



