214 IOWA ACADEMY OF SCIENCES. 



The lower one of the two embryo cells now divides 

 longitudinally. Just how further divisions of the embryo 

 occur, it has not been possible yet to determine, because 

 the sections were cut in an obliqne plain. 



The oldest embryo sectioned is shown in Fig. 19. 



WORKS CONSULTED. 



1. Strasburger, E.. Die Angiospermen und die Gym- 



nospermen. 1879. 



2. . Zellbildung und Zelltheilung. 1880. 



3. Guignard, M. L. D'Embryogenie Vegetale Com- 



paree. 1st Memoire. Legumineuses. Ann. Sci. 



Nat. Bot, VI, 14:5-166. 1881. 

 . Sur Le Sac Embryonaire Des Phaner- 



ogames Angiospermes. Ann. Sci. Nat. Bot. VI, 



i:i:136. 1882. 



3. Webber, H. J.. Pollen Tube of Zamia. Bot. Gaz., 



23: 453-459, also ^-i: 16-22, 225-235. 1897. 



. Xenia, or the immediate effect of pollen 



in Maize. Bull. U. S. Dept. of Agvl., Div. of Veg. 

 Path. andPhys. 22. 1900. 



4. Vines, S. H.. Student's Text Book of Botany: 431- 



462. 1896. 



5. Strasburger, Noll, Schenck, and Schimper.. Lehr- 



buch der Botanik fur Hochschulen, 389-393. 1894. 



THE THISTLES OF IOWA, WITH NOTES ON A FEW 

 OTHER SPECIES. 



BY L. H. PAMMEL. 



I have for some years been interested in a study of our 

 thistles. During my study in St. Louis I had occasion to 

 examine the rich collections of the Gray Herbarium, Har- 

 vard University, as well as that of the Engelmann Herba- 

 rium and the Missouri Botanical Garden, besides a consid- 

 erable collection in the Parry and I. S. C. Herbaria. I 

 should not attempt the publication of only a partial paper 



