34 FARLOW ON THE GYMNOSPORANGIA 



certainty to occur at all, and where, at least, the common Roestelia on apple leaves is cer- 

 tainly not R. cancellata. 



At first sight, then, we would not admit the correctness of Oersted's views with regard 

 to particular species without further inquiry, and one naturally resorts to artificial cultures. 

 These may consist in sowing the germinating sporidia of the different species of Gymno- 

 sporangium either on leaves of different Pomeae, kept moist under bell-glasses, or on the 

 young plants themselves. The former mode is more convenient, but has this objection 

 that, although after sowing sporidia on the leaves spermogonia may appear in from about 

 eight days to a fortnight, yet the interval is so great between the production of sperm- 

 ogonia and the development of the aecidia, from which alone the species can be with cer- 

 tainty determined, that the leaves, kept in a moist place, are almost sure to be destroyed 

 by moulds before the aecidia have developed. Cultures made with the young plants 

 themselves permit the development of the aecidia, but for mechanical reasons they 

 are less easy to manage, and one is also obliged to keep a series of plants on which no 

 sowmg has been made, in order to make it comparatively sure that the mycelium of the 

 Roestelia was not in the plants before the cultures began. I have, as a rule, made use of 

 leaves only becaxise the amount of space at my control was limited, and because it was 

 difficult for me to procvire young plants of some of the species required for cvdtures. 

 The season of the year when the subject must necessarily be studied, the latter part of the 

 spring, is, moreover, one when numerous occupations prevent my devoting as much of 

 my time to the cultures as I should like. 



In the spring of 1875, I procured two plants of Amelanchier canadensis about a foot 

 high, and sowed upon the leaves the sporidia of G. macropus which were seen by micro- 

 scopic examination to be in good condition. Nothing resulted from it. I was led to 

 begin with this experiment because the most striking Gymnosporangium in the region of 

 Boston is G. rnacrojMS, and one of the most prominent Roestehae is H. hotryajntes which 

 grows only on Amelanchier, and both species are peculiar to America. Since 1875 I have 

 repeatedly made attempts by cultures to demonstrate the connection between our different 

 species. The species with which I have experimented are G. macropus, G.fuscum var. 

 glohostmi, ^ G. Ellisii, G. hisej^tatum, and G. clavipes, all common near Boston. 



I. May, 1876. 



G. clavipes sown on 6 Amelanchier leaves. No result. 



G. Ellisii on 6 apple leaves, three Amelanchier leaves, and two leaves of Crataegus 



tomentosa. No result. 

 G. macrop)us on 3 leaves of C7'afaegns tomentosa, 6 apple leaves, 4 Amelanchier leaves. 



Spermogonia formed on one leaf of C. tomentosa. 

 G. (jlohosum on 3 leaves of Crataegus tomentosa, 3 of Amelanchier, and 3 of apple. 



Spermogonia appeared on all the leaves of Crataegus. 



II. May, 1876. 



G. macrojms on two small pear seedlings. No result. 



G. glohosum on one young plant of Crataegus oxyacantha. No result. 



III. June, 1876. 



G. glohosum on 5 leaves of Crataegus tomentosa. No result. 

 G. macropus on 3 leaves of Amelanchier. No result. 



1 For sake of brevity tliis form is given under the name of G. glohosum in the following tables. 



