THALLOPHYTA: FUNGI 145 



of the cup (Fig. 119/)). The chains consist of alternating spores and 

 sterile cells, the latter disintegrating. The aeciospores cannot infect the 

 barberry. Instead, carried by the wind, they infect wheat plants during 

 the late spring and summer, thus completing the life cycle. 



The cells of the mycelium produced by the basidiospores on the bar- 

 berry, as well as the spermatia, are uninucleate, but the aeciospores are 

 binucleate. The binucleate condition appears to arise by the spermatia 

 coming in contact with special receptive hyphae of the opposite sex. 

 These extend from the basal cells of the young aecium to the orifice of 

 the spermogonium, through which they project. A spermatium enters a 

 receptive hypha and passes down into the basal cell, which then becomes 

 binucleate. Each binucleate basal cell gives rise by repeated division to 

 a chain of aeciospores. The binucleate condition is carried over by the 

 aeciospores to the mycelium on the wheat and to the uredospores and 

 young teliospores produced by it. 



The fusion of the two nuclei in each cell of the teliospore introduces the 

 uninucleate condition. When the teliospore germinates, the diploid 

 nvicleus in each of its cells undergoes two successive meiotic divisions that 

 result in the formation of four haploid nuclei. Each of the four cells in 

 the basidium receives one of these nuclei, which then passes into a 

 basidiospore. The uninucleate basidiospores, being haploid, produce a 

 haploid mycelium on the barberry. Thus, although there is an alternat- 

 ing haploid and diploid phase in the life history of Puccinia, the latter is 

 not initiated by a nuclear fusion, as in most plants, but by the coming 

 together in the same cell of two nuclei that retain their identity through- 

 out a large number of cell divisions. Eventually the nuclear fusion 

 occurs, but is then followed by the reduction divisions, which mark the 

 beginning of the haploid phase. 



Other Rusts. The wheat plant is attacked not onl}^ by Puccinia 

 graminis, its most destructive rust, but by several related species. One 

 of these is Puccinia coronata, whose alternate host is the buckthorn 

 {Rhamnus); another is Puccinia rubigo-vera, which produces the aecial 

 stage on blueweed (Echiuni) and other Boraginaceae. All three species 

 may attack other grasses than wheat, such as barley, oats, rye, and 

 various meadow grasses, producing the same morphological type of 

 mycelium and spores on each kind of grass but a different physiological 

 strain. 



Many rusts have a shorter life cycle than Puccinia graminis. All rusts 

 produce teliospores and these always give rise to basidia and basidio- 

 spores, but one or more of the other spore forms may be missing. Thus 

 the aecia may be omitted, the uredospores, the aecia and spermatia, or 

 the aecia, spermatia, and uredospores. If a rust requires two different 

 and unrelated hosts to complete its life cj^cle, it is said to be heteroecious; 



