22i 



HAIIDWICKE'S SCIENCE-GOSSIP. 



destroys the matrix on wliicli it grows, and causes 

 the leaves to putrefy and dry up. Perfect specimens 

 are seldom ' met with on potato stems ; but the 

 destructive mycelial tlu'cads descend them, and so 

 reach the tuber. The>tcm now, like the leaves, 

 rapidly rots, and falls upon the earth an offensive 

 mass. So rapid and fatal is the growth of this 

 fungus, that in a few days it will spread from 

 plant to plant over a large tract, and in less 

 than a week turn every stem and leaf in the 

 field to one lotten mass. Within these diseased 

 stems' are often found black masses of hardened 



Fig. 153. Peronospora infestnns. Five days' growtli from 

 a spore, enlarged 400 diameters. 



threads, which are believed to be the mycelial fila- 

 ments, in a resting and highly condensed but still 

 living state : these black threads have been described 

 under the name of Sclerotinm varinm. Another form 

 of this substance, very common just under the bark 

 of old trees, has been described under the name 

 of RMzomorpha ; this is probably the mycelium of 

 some Tohjporus in a high state of condensation : 



similar threads have also been found on the wood- 

 work of old coffins. Returning to the young con- 

 dition of the potato fungus, we see it five days old 

 in fig. 153, where the distance from A to B shows 

 tiie thickness of the potato-leaf itself, magnified 

 400 diameters : A is the upper surface of the leaf, 

 and B the lower. The mycelial tlireads or spawn, 

 C, may be seen ramifying amongst the cellular 

 tissue of the leaf, whilst the fertile thread is shown 

 emerging through a breathing-pore, or stomate, D, 

 and branching and bearing (at present) immature 

 spores at E. 



Fig. 154. Peronospora iiifestans, enlarged 150 diameters. 



It is almost impossible to conceive of anything 

 which could have a more damaging effect upon a 

 plant than such a growth as this ; for, leaving out the 

 destructive nature of the mycelium within the leaf, 

 the whole of the leaves' mouths, or breathing-pores, 

 soon become completely choked up. It is some- 

 what analogous with a bad attack of croup in the 

 human subject, with the addition of an external 

 growth. Fig. 15i represents the mature condition 

 of the fungus enlarged to 150 diameters only : here 



