148 MORPHOLOGY OF ANGIOSPERMS 



cates that the interval between pollination and fertilization in 

 any given species may be dependent upon temperature. In the 

 case of Monotropa, under normal conditions fertilization takes 

 place about five days after pollination ; but by lowering the tem- 

 perature the interval is lengthened, and at 8-10 C. fertilization 

 is prevented. In Shibata's experiments it was shown that 

 light, atmospheric pressure, and mechanical injury seem to 

 exert no influence upon fertilization and subsequent phenomena, 

 but that the structures of the embryo-sac are very sensitive to 

 temperature. 



In a long pollen-tube, or in one that persists for a long time, 

 it is common to observe the formation of successive cellulose 

 plugs (Prop fen) that shut off the growing tip, with its cells 

 and nuclei, from the cavity behind, as fully described by Stras- 

 burger 4 and Elfving. 6 Sometimes the plugs are so large and 

 persist in such a series that they become conspicuous objects, 

 as in Gymnadenia conopsea (Marshall-Ward 7 ), Campanula 

 americana (Barnes 9 ), Barcodes sanguinea (Oliver 11 ), etc. In 

 such forms as the Amentiferae and others, in which the tube 

 and its contents remain imbedded in the stylar tissue for a 

 period varying from one month to over a year, the tip of the 

 tube is cut off by a plug, its wall thickens, and it passes into 

 what might fairly be called an encysted condition, as suggested 

 by Miss Benson 15 in connection with Carpinus. 



The branching of pollen-tubes, so conspicuous a phenome- 

 non among Gymnosperms, is also found among certain Angio- 

 sperms. Hofmeister 3 observed branching tubes among Mono- 

 cotyledons in Potlios longifolia and Hippeastrum aulicum. 

 Among the Amentiferae it seems to be very common, Miss 

 Benson 15 observing forking tubes in several of the genera 

 (Corylus, Carpinus, etc.) she studied, and in Quercus a cluster 

 of short branches at the end of the tube ; while ^awaschin 20> 3( 

 states that the tubes of Juglans and Ulmus branch profusely, 

 and recently a similar branching has been noted by Billings 6 

 in Gary a (Hicoria). Zinger 31 also described the pollen-tubes 

 of the Cannabineae as ending in numerous swollen sac-like 

 branches. The breaking up of the tip of the tube into short 

 branches is doubtless a common phenomenon, probably associ- 

 ated with the rhizoidal habit, but free branching seems to be 

 characteristic chiefly of chalazogamic forms. 



