310 A TEXTBOOK OF THEORETICAL BOTANY 



before cereal grain is sown means that these secondary spores are lying in a 

 position entirely favourable for infection. In the case of Loose Smut, 

 infection generally occurs only through the flowers, for the fungal mycelium 

 cannot penetrate the tissues of the mature plants. On the other hand, 

 the young seedlings can be infected up to the time when the first leaf is 

 produced. Thus secondary spores lying in the soil may attack and infect 

 young seedlings. In the case of Covered Smut the chlamydospores are 

 already present in the grain, and if the grain is sown as seed a dormant 

 mycelium may resume activity as the seed germinates and pass into the 

 young shoot ; or infection may take place from outside through spores 

 already liberated into the soil. 



It is worthy of note that the chlamydospores, except in U. avenae, only 

 retain their vitality for a few months, after which they cannot cause infection. 

 Thus if the weather is wet at the flowering time of the cereal, most of the 

 chlamydospores will be washed to the ground and the grain from that 

 crop if used for seed will give rise to a new crop comparatively free 

 from Smut. 



Once penetration has occurred the mycelium makes its way to the grow- 

 ing apex of the plant and maintains its growth so as to keep pace with that of 

 its host. These hyphae are intracellular and appear to cause little or no 

 disturbance to the host plant. Hyphae in the older parts disappear, and 

 it is only in the younger parts of the stem that active living hyphae 

 can be found. When the flowers begin to form the mycelium enters the 

 ovary where it forms a dense mass within the tissues, which are thereby 

 destroyed. 



After this has been completed the hyphal wall becomes gelatinous and 

 swells up into segments which are then cut off by septa. These segments 

 then give rise each to a single chlamydospore whose wall thickens and hardens 

 as it separates from its neighbour, and the deep purple colour of the mature 

 chlamydospore is assumed. Alternatively chlamydospores can be budded 

 out laterally from the hyphae. 



Little is known about the actual fusion of the mycelia produced from the 

 basidiospores. Normally it would appear to take place within the host 

 tissue, so that the cells of the resulting mycelium are binucleate as in the 

 Rusts, union being effected during the maturation of the chlamydospores. 

 Unfortunately the cytological details have not been critically worked out. 

 In certain allied species, such as U. violacea, for example, the fusion has 

 been observed. It may be brought about by the union of separate basidio- 

 spores which conjugate by means of a tube, through which the nucleus 

 of one spore passes into the other (Fig. 303). Union may also occur between 

 a basidiospore and a cell of the basidium. In either case the infecting 

 mycelium is binucleate, and it is only from such a binucleate mycelium 

 that chlamydospores can be developed. In Ustilago, therefore, the only 

 uninucleate stage is the basidium. 



Certain instances have been described in which there is union between 

 basidiospores or mycelia of different species of Smuts. There is reason to 



