(397) 



spore-columns originate as branches from these large multinu- 

 cleated cells. 



It has been suggested by a number of investigators that this 

 nuclear history represents an alternation of generations. Ac- 

 cording to this view the gametophytic generation is uninucleated, 

 has the reduced number of chromosomes, and beginning with the 

 basidiospores gives rise to the pycnia with their pycniospores. 

 The sporophyte is binucleated, has the double number of chromo- 

 somes, and beginning with the aeciospores gives rise to the telia 

 with their teliospores (also to uredinia with their urediniospores 

 when they appear). The gametophytic number of chromosomes 

 is restored by a reduction division when the teliospore forms the 

 four basidia in germination. 



2. Distribution and Relationships 



Problems 

 It is at once evident that the same environmental factors which 

 affect the distribution of the higher plants cannot have the same 

 influence, at least they cannot operate in the same way, on the 

 distribution of parasitic plants. It is also so apparent, as to scarcely 

 require mention, that the geographic distribution of the obligate 

 parasite is limited by the distribution of hosts and cannot be 

 found outside of their range. But as to whether the distribution 

 of the parasite is coextensive with the range of the hosts, as it might 

 be provided there were no other operating factors than the 

 mere presence or absence of the hosts, is quite another question. 

 In order to throw some light on this problem and to discover, if 

 possible, the existence and nature of other factors, the distribution 

 of the species of Gymnosporangium has been worked over. In this 

 genus, where all but one of the known species are heteroecious, 

 there is a possibility of gaining some idea (i) of the dependence 

 of the fungus upon its two sets of hosts, (2) of the relation of the 

 distribution of these sets of hosts to each other, and (3) of the 

 comparison between that relationship and the distribution of the 

 fungus. This opportunity is one peculiar to this genus because 

 of its lack of repeating spores, i. e., spores which are able to grow 

 again on the same host and reproduce the phase from which they 

 were derived, and of the consequent absolute necessity for the 

 association of the two sets of hosts as a condition in the distribution 

 of the fungus. 



