ORDER UREDINALES (tHE RUSTS) 393 



The binucleate aeciospores are capable of remaining viable for a long 

 time and can be carried great distances by the wind and still cause infec- 

 tion. Falling upon a suitable host plant the spore germinates in a drop of 

 water (dew, rain, etc.) forming a stout germ tube which usually seeks out 

 and enters a stoma. Pady (1935) showed that in Gymnoconia peckiana the 

 germ tube penetrates the epidermis directly as do the germ tubes from 

 the sporidia, but this seems to be an exception. Just within the stomatal 

 opening the hypha enlarges to form a "substomatal vesicle" containing 

 many nuclei from which arise hyphae of dicaryon cells which penetrate 

 the host tissue in various directions, growing intercellularly and sending 

 haustoria into some of the cells between which they pass. Since the normal 

 aeciospore is binucleate this mycelium arising from it is dicaryotic and 

 has two nuclei in each cell, one descended from that of the sperm and 

 one from that of the parental monocaryon mycelium. Usually the size of 

 the infected area from one aeciosporic infection is limited, i.e., the fungus 

 in this phase is not capable of indefinite growth. When its full extent of 

 development is nearly attained a subepidermal mass of hyphae is formed 

 making a pseudoparenchymatous layer of dicaryon cells from which grow 

 short upright two-celled branches. These raise and eventually rupture the 

 epidermis, thus producing a uredial sorus or uredium. The outermost of 

 the two cells enlarges to form a binucleate urediospore while the other 

 elongates to form its long stalk. The urediospores are colorless or more 

 usually yellow to orange red in color, mostly finely verrucose or echinu- 

 late. There are several germ pores for each urediospore and their number 

 and position on the spore are of great assistance in the identification of 

 the various species. They break loose from the stalk and like the aecio- 

 spores may be carried long distances by the wind. They germinate and 

 infect the host through the stomata in exactly the same manner as do the 

 aeciospores. As long as the host plant has not passed a certain stage of 

 development these new infections produce other crops of urediospores. 

 As the host plant becomes more mature tehospores begin to appear, often 

 at first intermingled with the urediospores in the same sorus, but eventu- 

 ally in sori containing only teliospores. They arise from the same type of 

 pseudoparenchymatous subepidermal mycelium as do the urediospores. 

 From this basal layer there grow upward series of dicaryon cells. Some of 

 these may differentiate into stalk cells and teliospores or all may become 

 teliospores. Malengon (1936) observed the production rarely of teliospores 

 in the aecial sori of Puccinia atropae Mont. In some of the most primitive 

 rusts no basal layer is formed and the tehospores are produced singly or 

 by twos or threes as enlarged cells of the mycelium in the interior of the 

 leaf as occurs in Uredinopsis. The young teliospores, whatever their shape 

 or location, are at first binucleate. The two nuclei unite to form a diploid 

 nucleus. Usually the teliospore becomes thick-walled (mostly with one or 



