90 MANUAL OF BOTANY 



This plant and those Hke it, which thus inhabit two host- 

 plants in their life cycle, are called heteroecious. 



There are many of these rusts, which vary in several ways. 

 -The conipomid teleutogonidia sometimes have more than two 

 cells. Sometimes the secidium form inhabits the same host as 

 the uredo form, when the fungus is said to be autcecious. 

 Sometimes again one or the other phase of the life cycle does 

 not occur. 



Though the secidium is held by some to correspond to the 

 ascocarp or ascogonium of the Ascomycetes, no sexual process 

 causing its development has been discovered. 



The rusts constitute the family of the Uredinese, named from 

 the uredo form of the fungus. 



The other family included in this sub- class is that of the 

 Ustilagineae or smuts. They differ from the Uredineae in not 

 producing secidia. Like them the mycelium infests the tissue 

 of grasses, ultimately making its way into the ovary of the 

 latter, replacing its normal contents by a mass of black resting 

 gonidia. These when liberated germinate, forming a small 

 promycelium on which sporidia of curious shape are borne. These 

 sporidia sometimes coalesce in pairs, sometimes remain separate. 

 On germination they either produce the original form of my- 

 celium, or give rise to another promycelium bearing sporidia. 

 The latter then produce the first form. 



Sub-Class YI. — Basidiomycetes. 



This group includes the Mushrooms, Puff-balls, &c. It is 

 characterised by the entire absence of sexual reproductive 

 organs. The mycelium, which is incompletely septate, burrows 

 throughout the substratum on which the fungus grows, and 

 sends up above the surface large fructifications which bear the 

 spores or gonidia. These are formed by abstriction from sjDecial 

 cells known as basidia, which give the name to the group. The 

 form of the fungus is always that of a potential gametophyte, 

 but the power of producing gametes is absent. 



The distinguishing feature of the group is the large sub-aerial 

 fructification or gonidiophore. Several* varieties of this are 

 found, of which the most familiar is that produced by the conunon 

 mushroom {Agaricus campestris) {Jig.85S). A conical or pear- 

 shaped outgrowth appears upon the mycelium, consisting of a 

 number of hyphae closely cohering together and growing at 

 their apices. The direction of the growth soon changes, the 

 hyphae spreading outwards to form a prominent head which 



