Dodge: The Genus Gymnosporangium 191 



common method in this genus of the rusts. The hosts attacked 

 by the six species studied represent three species and two genera 

 of conifers. The epidermis of the leaves and young stems is 

 rather heavily cutinized, and the galls or stems upon which the 

 sori of some of the species develop are covered with layers of 

 cork of considerable thickness. After the cork (or epidermis) 

 has been lifted up or broken open and the pressure on the mar- 

 ginal hyphal mass has been removed we might expect to find that 

 the later-formed spores (at the margin) arise directly from the 

 terminal cells. If the terminal cell becomes disorganized in 

 response to a particular need for some sort of space-making unit 

 it would not be necessary for every terminal cell in the primor- 

 dium to degenerate. It is sometimes difficult to ascertain just how 

 the spores are formed at the margin of a nearly full-grown sorus. 

 I have as yet found no evidence that they arise at any time from 

 terminal cells. It is not expected, of course, that this rule will 

 apply to all other species of Gymnosporangium. 



If the terminal cell represents a morphological unit in the pri- 

 mordia of the rusts, a unit not a basal cell, but one having either 

 the space-making function or some unknown function we ought 

 to find such units in other genera. The presence of sterile 

 tissue in the accidium primordium in addition to the sterile cells 

 above the gametic cells has been noted by those who have studied 

 this spore form. The morphological or phylogenetic significance 

 of these peculiar cells certainly has not been overemphasized, and 

 it would be interesting to know just to what extent space-making 

 requirements could lead to degeneration of tissue in the primordia 

 of all spore forms, uredo sori and spermogonia included. 



Columbia University, 

 New York City. 



Explanation of Plates 



The drawings were made with the aid of the camera lucida. Zeiss No. 4 

 eyepiece and i/i2th oil immersion lens were used in making Figs. 1, 2, 4-8. 

 No. 5 eyepiece was used for Figs. 3, 9 and 10. Reduction about one fourth 

 for Plates 10 and 11 and about one third for Plate 9. 



Plate 9, Gymnosporangium macropus 



Fig. 1. Beginning of a sorus primordium at the center of a depression or 

 pit in the gall. 



