On the Diseases of Wheat. 



3 



and can only be just distinguished by the highest powers of the 

 microscope. Many of these kind of fungi hve beneath the scarf- 

 skin, or epidermis^ and within the very substance of certain plants. 

 In the progress of their growth, they raise blisters under the 

 epidermis, and, when arrived at maturity, they burst through it, 

 and then form spots or irregular blotches of various colours, which 

 are frequently orange, brown, or bla^k. These spots (or sori) 

 are masses of fructification, and are surrounded by the tattered 

 edges of the ruptured epidermis. A vast number of these fungi 

 are known to botanists. Like parasitic animals, they are re- 

 stricted in their powers of attack, being able to live on certain 

 species only, and even on particular parts only of particular indi- 

 viduals of these species. There is often a strong general resem- 

 blance between many of them ; but a naturalist will readily detect 

 such important differences between two fungi which may infest 

 distinct species of plants, that he is compelled to consider them 

 also as species distinct from each other. Thus it happens in the 

 animal kingdom, that different species of flea, and different 

 species of lice, can exist only on particular species of quadrupeds 

 or birds. The flea which infests dogs is distinct from that which 

 annoys man. So also with these parasitic fungi; some are 

 restricted to one species of plant, some to another : but, generally 

 speaking, most of them are capable of living upon more than one 

 species of the same genus ; where, of course we might expect the 

 resemblance in all points to be very close. Some fungi confine 

 their attacks to the seed, others to the stem or leaves, and some 

 even to one side only of the leaves. One of those which attack 

 wheat lives only on the grain, another more particularly attacks 

 the short stalk {pedicel) on which each flower is seated, whilst 

 three of which we are about to speak are restricted to the straw, 

 chaff, and leaves ; but all five live at first beneath the epidermis, 

 and not upon it. In this respect they bear a close analogy to those 

 parasitic animals which live within the bodies of other animals, 

 some immediately beneath the skin, others in the intestines, and 

 others again within the very substance of the muscle. It is the 

 extraordinary minuteness of the sporules (or seed-like bodies) of 

 these fungi, which allows of their being absorbed by the roots, and 

 probably also through the pores of the stem and leaves of plants ; 

 and then they are conveyed by the sap to the various parts where 

 they are enabled to germinate, grow, and fructify. The sporules 

 of fungi appear to be every where dispersed through the atmo- 

 sphere, ready to germinate wherever they may find a dead or 

 living subject in a condition suited to their attack. Common 

 mouldiness, for instance, which so readily forms on many sub- 

 stances in moist situations, is the most familiar example of the 

 inconceivable numbers in which the sporules of a minute fungus 



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