642 Journal of Agricultural Research voi. xv, no. i» 



time moving toward the germ tube. At present no data can be given on 

 the young mycelium from the uninucleate or binucleate sporidia, and 

 this comparison must remain impractical until the process of sporidial 

 germination and infection of the pine can be more closely followed. 



NUCLEAR PHENOMENA IN VEGETATIVE MYCELIUM IN PINUS STROBUS 



The uninucleate mycelium produced in the bark of Pinus sirobus 

 following infection by the sporidia has already been described. The 

 nuclei are completely organized, with a definite nucleolus, chromatin 

 network, and membrane (PI. 57, HH, II). The chromatin appears to be 

 more or less definitely centered on one point on the nuclear membrane 

 where the centrosome is located. This condition of the arrangement of 

 the chromatin elements of the nucleus will be referred to as polarization, 

 and will be discussed in connection with its appearance in other stages 

 in the cytological cycle. Few cases of vegetative division have been 

 observed in the vegetative cells, but the stages of the process which have 

 been seen indicate that there is a t5^pical rust spindle and that the divi- 

 sion is comparable to the vegetative divisions described by Olive (41). 



NUCLEAR PHENOMENA IN PYCNIUM 



The nuclei in the pseudoparenchyma layer from which the short 

 branches bearing the pycnial sporophores arise are quite similar to those 

 of the vegetative mycelium in shape, size, and organization, but they are 

 colored more intensely with hematoxylin and the violet of Flemming's 

 triple stain. The nucleus of the pycnial sporophore is relatively large 

 for the size of the sporophore (PI. 58, A). In its resting state the chroma- 

 tin is scattered throughout the whole nucleus in minute granules, which 

 are rather difficult to stain clearly. Each nucleus has a definite centro- 

 some indicated by the local condensation of the chromatin (PI. 58, A, a). 

 The pycniospore (PI. 58, A, b), which swells to its full size on the tip of 

 the sporophore, receives one of the daughter nuclei resulting from the 

 division of the sporophore nucleus (PI. 58, A, d). The exact details of 

 the process were not definitely followed, on account of the small size of 

 the pycniospores and the narrow passage from sporophore to spore. The 

 mature subpyriform pycniospore has the typical structure of correspond- 

 ing forms in other genera of the rusts. A relatively large nucleus is sur- 

 rounded by a small amount of cytoplasm (PI. 58, A, b). 



NUCLEAR PHENOMENA IN ^CIUM 



In the layer of fertile cells and in the mycelium below this layer the 

 nuclei are somewhat larger (PI. 58, H) and more readily stainable than 

 those of the deeper vegetative mycelium (Pi. 57, HH). All the nuclei 

 in this region of the layer and below it exhibit the phenomena of polariza- 

 tion, although the fact may not be evident unless the profile view can be 

 seen. The same condition persists in all other nuclei in or near the vari- 



