Comparative Physiology. 203 



On the question whether fungoid growths on leaves, &c.j 

 may be regarded as degenerations merely of the tissue on 

 which they grow, he quotes the opinion of linger, that blight, 

 mildew, smut, &c., are diseases of the stomata; the Exan- 

 themata (eruptive fevers) of vegetables. " The state of our 

 knowledge at present is not such as to enable us to decide 

 whether, as reproduction seems only to be a peculiar form of 

 nutrition, if its regular form of developement be prevented, it 

 may not give origin to beings of more simple organisation, and 

 these fungi be formed in place of higher forms of existence." 

 " Many substances once thought fungi," he says, " are now 

 found to be only accidental and irregular expansions of the tissues 

 of flowering jalants, become deformed through growing in the 

 dark, as in cellars, caverns, &c. Animals and plants are both 

 liable to the growth of fungi within their bodies." 



To say that degenerations of tissue might give rise, when 

 prevented from higher developement, to beings of more simple 

 organisation, is not so indicative of divine wisdom, as to say 

 that the diseased morbid matter of the tissue formed the 

 food of the germs of an inferior being. The germs of 

 these fungi have been generally thought to give rise to blight, 

 mildew, &c. They imdoubtedly appear as diseases of the 

 leaves, whether of the stomata or not. Whatever checks 

 growth appears to cause a morbid diseased state of the cuticle 

 on all the places where growth is most active. When beeches, 

 especially large plants in hedges, are stoj)ped in their growth 

 by spring frost and drought afterwards, the diseased state of 

 the leaves and young shoots appears in an exudation of sap, 

 which attracts innumerable swarms of aphides. If the weather 

 sets in moist, warm, and encouraging growth, after the check 

 by frost, the disease does not extend so far. It is the same 

 with the larch and some other plants, when attacked by frost 

 and subsequent drought. Thorns do not suffer from frost, but 

 they are exceedingly fond of moist wet weather, and in moist 

 summers I have always observed them to make most growth. 

 In dry weather, if continued long, they never fail to set up 

 in growth, and the whole quarters of that plant in the nursery 

 grounds become white in the foliage with mildew. It has 

 been generally thought to arise from the imperceptible germs 

 of mildew floating in the atmosphere, which, from the diseased 

 state of the leaf, have (like the insects attracted in the case of 

 the other plants) found here their proper food, without which 

 their developement could not take place. If this were an erup- 

 tion from the stomata, it should appear most on the under side 

 of the leaf ; but the reverse is the case ; it appears princij)aUy on 

 the upper side of the leaf; and, when refreshing showers com- 

 mence and continue for any length of time, it is washed oft" the 



