NOTICES OF BOOKS AND MEMOIRS. 28 



after a time by the sucking organs, or iiaustoria, which become 

 formed by the older branches. Each bores through the wall of 

 the cell with which it is in contact, and by means of the perforated 

 passage obtains as food the contents of the leaf-cell. As growth 

 proceeds in all directions from the ]3oint of entrance of the 

 germinal tube — i. e., a stoma — the leaf-cells first attacked and 

 injured are evidently those nearest this central point, and the 

 destruction of tissue proceeds in a centrifugal manner, pari j^ctssu, 

 with the spread of the destroying mycelium. The injured cells 

 become paler in colour as their contents become altered and 

 destroyed, and thus shine through the outer layers of the leaf 

 with a paler hue than the remainder of the tissue : the yellowish 

 circular spot thus produced is the first indication to the naked 

 eye of the damage done to the leaf— it is the so-called 'pin-spot.' 

 As the destroying mycelium extends itself further into the tissues, 

 its course is marked by dying cells, and a circular spreading of the 

 pale disease patch is obvious to the outward observer. From the 

 same cause the discoloration appears at a later date on the upper 

 surface of the leaf, as the branches extend there. 



When a vigorous, centrifugally spreading mycehum has thus 

 been formed, the older portions at the centre commence to form 

 spores : these are produced in compact groups from the substance 

 of certain processes which are formed by aggregations of branches 

 of the mycelium forced through the stomata. The first formed 

 spore-groups appear at those stomata which lie close around the 

 point of origin of the mycelium, and they are followed by others 

 successively protruding through stomata further and further away 

 from this point : thus, the production of spore-bearing heads also 

 takes place in a centrifugal manner, and successive cu'cles of them 

 become arranged around those first formed. Each spore-bearing 

 head is capable of producing large numbers of spores, successively 

 budding forth during the period of activity. After a stock of 

 spores has commenced to accumulate, falling off as they become 

 ripe, and adhering to the leaf as the yellow or orange-coloured 

 ' rust powder,' the completely exhausted leaf- cells, which have 

 been destroyed by the mycelium, turn brown and decay, and in 

 place of active, translucent, living cells, we find a mass of empty, 

 shrivelled, useless, and discoloured vesicles. From what has been 

 said above it is evident that the resulting brown dead patch, seen 

 from without, necessarily commences in the centre, and spreads in 

 a ch'cular manner as before. Under certain circumstances, the 

 second form of spore is produced later by the old spore-bearing 

 heads, and shortly afterwards the growth ceases. In most cases, 

 however, the attacked leaf falls before this, especially when many 

 ' disease spots ' have become established in its tissues. 



The above is a short account of the succession of phenomena 

 presented by the fungus on the coffee, from the germination of the 

 papillate spore to the production of many similar spores from the 

 adult mycelium ; and it must be noted that each one of these spores 

 is capable of reproducing the same cycle of phenomena, x^rovided it 

 meets with proper conditions of development. In this way the 



