32 ACOTYLEDONEJE. 



Fig. .9. 



Pucinia graminis on the straw of wheat. 



Its prevalence in some seasons, can only be ac- 

 counted for by supposing-, that the seeds are conveyed 

 from the perfect plants by the wind, and, lodging- in 

 the pores or cells or cuticle of the straw, veg-etate and 

 come to maturity. It is remarked of the Fungi, as 

 well as of many of the other orders, that they affect 

 peculiar substances as their habitat. Some are only 

 met with on particular kinds of dead or dying- trees ; 

 many luxuriate on the bark, others on decayed roots, 

 and several occupy the solid timber in the various 

 stages of its decomposition. Many attach themselves 

 to particular kinds of rock or earth, whilst others 

 prefer the leaves of living plants. 



The growth of this division takes place by the 

 inflation of the cellular structure ; commencing, as 

 before observed, at a little distance from the centre, 

 and proceeding outwards, Fig. 5. Whether the sub- 

 stance be carnose, membranaceous, or foliaceous, the 

 exterior is invariably covered with a dense film or 

 cuticle, which in many cases parts readily from the 



