LEAF AND SEEDLING DISEASES 175 



considered to be vestigial male cells (spermatia) though by 

 some authors they are thought to be conidia or spores 

 (pycnopores). They have never been known to germinate, 

 and in any case they are now functionless. 



This very brief description of the life-history of a hete- 

 roecious rust is only intended to assist those unacquainted 

 with the group to follow the account of the two rusts which 

 infect the larch. A more comprehensive account will be 

 found in any botanical text-book or in Grove (1913). 



Melampsoridium betulinum, K\eb. ( = Peridermium laricis). 

 As the heteroecious rusts have frequently been known as 

 different fungi on their two hosts, they have commonly been 

 designated by two names. For instance, Peridermium laricis 

 is an aecidial form which is found on larch needles. The 

 uredo- and teleuto-form which occurs on the birch has also 

 been called Melampsoridium betulinum (or formerly Melamp- 

 sora betulina, Desm.). As it is cumbrous to keep two different 

 names for one fungus, it has become the accepted practice to 

 designate a rust whose complete life-history is known by the 

 name applied to the uredo-form. Consequently the name Peri- 

 dermium laricis, by which this rust is most generally known 

 to foresters, has to give place to Melampsoridium betulinum. 



Fig. 72 shows the general appearance of this rust on the 

 larch. The aecidia are very conspicuous by reason of the 

 large white sheath which surrounds each pustule. This 

 sheath, which is called a peridium, is one cell in thickness, 

 and is shown enlarged in fig. 72, 4, and fig. 72, 7 shows the 

 arrangement of the cells of the peridium when a portion of 

 it is placed flat on a slide under the microscope. The 



DESCRIPTION OF FIG. 72. 



1. Photograph of twig of Larix Europea, with diseased leaves: () 



pseudoperidium with lacerate margin ; (b) unopened pseudo- 

 peridium ( x 4). 



2. Photograph of twig of ditto, with fused pseudoperidia ( x 4). 



3. Photograph of twigs of ditto (about natural size). 



4. A pseudoperidium showing lacerate margin ( x about 50). 



5. Spore in surface view ( x 1000). 



6. Spore in optical median section ( x 1000). 



7. Part of pseudoperidium ; the lower cells show the verrucose marking 



on the walls ( x 220). 



(After A. W. Borthwick and Malcolm Wilson.) 



