83 



THE COURSE OF THE POLLEN TUBE IN HOUS- 

 TONIA: A PRELIMINARY NOTE 



I5y Francis E. Li.ovd 



In 1902* I announced that in certain Rubiaceac, namely in 

 the genus Honstoiiia, the ovule is not supplicfl with an integu- 

 ment, realizing the '' nucellus 7iudns" of Schleiden, a condition 

 supposed erroneously by him to obtain in the Rubiaceae in gen- 

 eral. At present lloustotiia is the only genus of this family in 

 which this peculiar and unexpected condition has been announced 

 to occur, although it has recently been found by me that other 

 genera closely allied to Houstonia are similar to it in this regard. In 

 the paper above cited it was also shown that the course of the pollen 

 tube in other Rubiaceous genera, namely Richardsonia and Diodia, 

 is also of especial interest. In Ricliardsojiia pilosa, the species 

 studied, the pollen tube takes an intercellularcourse, the path being 

 constant in its direction. This is true also of /)^W/V^/'^;r.y. D. \lr- 

 giniana, on the other hand, offers a contrast in that for a part of 

 the path the tube moves freely in the ovarian cavity, though in a 

 direction in general similar to that in the other species studied. 

 The significance of this remarkable dissimilarity I have discussed 

 elsewhere f but it may be added that similar relations have been 

 observed by Longo in an entirely different group of plants, the 

 Cucurbitaceae. 



The fact that the pollen tube in some of the Rubiaceae is inter- 

 cellular in its mode of growth, coupled with the further fact that 

 in Iloiistoiia no micropyle is present, a condition due of course to 

 the absence of the integument, led me to the belief that the course 

 of the pollen tube in the latter also would be found upon exami- 

 nation to be intercellular. This hypothesis was strengthened 

 again by the similarity of the topography of the ovary in Ricli- 

 ardsonia, Diodia, and Houstonia, apart from the placental struc- 

 ture, together with the disposition of the ovules in the last named. 



* Lloyd, F. E. The Comparative Embryology of the Rubiaceae. Memoirs of the 

 Torrey Botanical Club, 8 : 27-112. //. 5-/J. 15 F 1902. 



fThe Pollen Tube in the Cucurbitaceae and Rubiaceac. Torreya, 4 : S6-9I. 

 Je 1904. Pertinent literature is here cited. 



