90 



and on their surfaces when the course is superficial. This con- 

 clusion is drawn from the circumstances that the pollen tubes of 

 Hiunulus and Cannabis, when growing upon a cover-glass in 

 humid air, are indeterminate in the direction of their growth. 



We may now turn to the facts concerning the behavior of the 

 pollen tube in the Rubiaceae. The plants which offer these are 

 Richaniso}iia pilosa, Diodia teres, and D. Virginiaiia. The o vary- 

 in RicJiardsonia is trilocular, in Diodia, bilocular. Jn all three 

 there is a single ovule in each locule, and this is inserted at the 

 top of the basal element of the partition which separates the 

 locules. This partition reaches to the middle height of the ovary, 

 and the separation of the locules from this point on is completed 

 by the stylar elements. The latter has a conductive tissue of 

 elongated cells, which extends to its point of fusion with the 

 basal element of the partition. Up to this point in all three the 

 course of the pollen tube is direct and endotropic. After this, 

 however, it is quite different, in Richardsouia and Diodia teres on 

 the one hand, and D. Virginiana on the other. The ovule in 

 Diodia Virginiana has a tract of epidermal conductive cells 

 which are cubical in form, on the outer surface of which is 

 secreted a mucilaginous substance. On the surface of these cells, 

 the pollen tube moves freely till it reaches the micropyle, which 

 it enters and traverses till it reaches the embryo sac. The con- 

 ductive tissue is confined to a strophiolar outgrowth of the ovule. 



In RicJiardsonia and D. teres, however, the ovules are provided 

 with a conductive tissue of deeply columnar epidermal cells, with 

 dente protoplasmic contents, and thick but soft and yielding 

 walls. There is, however, no mucilaginous .secretion to be found 

 upon their exposed ends. In these plants, the course of the 

 pollen tube is always {^Diodia teres) or nearly always [Rie/iaid- 

 sonia) comi^lctcly endotropic. The pollen tube grows between 

 the epidermal cells, as in the Cucurbitaceae, and follows a direct 

 path toward the micropyle at right angles to the direction of least 

 resistance. Aside from the latter fact, we have, it is seen, tzvo 

 closely related genera, and tzoo sfyeeies of one of these, in lohich the 

 behavior of the pollen tnbe is different, in that, in one species the 

 course is ectotropie, and in the ttco others, distributed in different 

 genera, the course is endotropic. 



