67 



Though the spots bear some resemblance to each other in all these 

 various situations, they vary more than does the form of the fungus 

 which produces them. This fungus is of a dark colour, somewhat 

 greenish-black, and appears in larger or smaller tufts on the dis- 

 coloured surface of the plant. Frequently these tufts are of micro- 

 scopic size, appearing to the unaided eye merely as minute specks on 

 the surface. Sometimes the tufts are run together in larger areas, 

 but this is not often the case. When run together in this rare manner 

 the entire growth presents a somewhat velvety appearance. The 

 elements of which these tufts are composed are represented in the 

 illustration on the previous page. 



A considerable number of fungi of this kind have been described as 

 growing on citrous hosts. They have been assigned to the genus 

 Macrosporium and the genus Alternaria, though it seems to me 

 probable that most, if not all, of them are of the form usually assigned 

 to the genus Alternaria, 

 and that when they have 

 been assigned to the genus 

 Macrosporium, it has been 

 without the knowledge that 

 the spores, which, upon 

 casual observation, appear 

 to be borne singly, are in 

 reality borne in chains. 

 Observation, unless care- 

 fully made, would often 

 lead one to the erroneous 

 conclusion that the spores 

 are borne singly, so easily 

 are they shaken apart, and 

 so almost invariably are 

 they washed apart in the 

 water in which they are, 

 as a matter of course, ordinarily examined. One is almost tempted to 

 think that some observers have never seen these spores growing upon 

 their stalks in a natural position, for they are frequently figured 

 upside-down a position in which they would naturally be arranged 

 by one who did not know their method of growth. It is my experience 

 that these fungi almost never fail to give rise to spores in chains in the 

 perfectly still atmosphere of the cultivation chamber. 1 cannot pre- 

 tend to anything like a complete knowledge of these large and almost 

 universally distributed form-genera, but certainly so far as my ob- 

 servation does extend, it points towards a more complete synonomy of 

 Macrosporium and Alternaria. 



I find that, according to their situation, these fungi vary much in 

 the size and form of their spores, and I think this fact has led to a 

 needless multiplication of specific names. Very small variations in 

 the form and size of the spores, especially if associated with a new or 

 different host or different part of the world, have been considered 

 sufficient to warrant the application of new names. Of course, by 



Fig. 78. Typical growth of 

 Alternaria. Spores of the 

 Altei-naria of the tomato 

 as found growing on the 

 fruit ; a, an aerial hypha, 

 at the top of which a 

 spore is just beginning to 

 form; 6, a young spore 

 before it has begun to 

 divide in the niurifonn 

 manner ; c, a young two- 

 celled spore; d, a young 

 four-celled spore; e, a 

 spore approaching ma- 

 turity; /, two spores borne 

 tandem ; g, h, i, three 

 spores borne tandem ; 

 .?, mycelium from which 

 the spore-bearing hyphae 

 spring. 



