1710 DISEASES OF VARIOUS CROPS 



7) In the primary phases the structure of the c^'toplasin of the cell 

 undergoes changes. The grains of chlorophyll are op the verge of disin- 

 tegration (chlorophyll disintegration phase) and at the same time the plas- 

 ma itself is granular. 



8) In the following phase the structure of the cytoplasm alters: 

 the chlorophyll grains are broken down, the whole mass of plasma seems 

 to be more granular and contains numbers of nucleoli (4 to 6) (nucleolar 

 phase). As a result of the disintegration of the chloroph^'ll the leaves turn 

 black. 



()) This phase is followed by another in which greater changes come 

 about. The plasmic granules gather together in certain parts of the cell, 

 especially in the palisade tissue of the parenchyma. The plasmic mass of- 

 ten lies within them. Sometimes the collection of granules is found in the 

 middle or it may lie towards the wall nearest to the epidermis. The nucleoli 

 are replaced by C3'toplasmic granules of irregular shapes (maturation 

 phase) . 



10) The plasma within the diseased cell from the very beginning 

 is of two kinds: there is the plasma of the host and that of the fungus. They 

 are intimately mingled and form a symbiotic association which, inherited 

 from the parent plant, spreads from cell to cell. The writer speaks of this 

 plasmatic association as « mycoplasma ». At a certain stage in the de- 

 velopment of the nurse-plant a struggle begins between the two, which 

 ends in a victory for the fungus. 



11) At this time the plasmic body leaves the cell and penetrates 

 into the intercellular spaces where it begins to form the m^xelium. The 

 plasma bores through the cell wall where the granules are specially thick 

 (mycelial stage). 



The plasmic mass becomes filamentous, the filaments being either 

 simple or branched, according to the size and shape of the intercellular 

 si^ace. If the plasmic mass makes its escape at the outer end of a pahsade 

 cell it becomes a fine thread which gets in between the epidermis and the 

 cells below. At the inner end one often sees a large vesicle entirely or part- 

 ly empty, its contents having been discharged into the filament. 



12) The development of the mycelium seems to proceed in two di- 

 rections; some remain thin and show separate nucleoli. Some of these break 

 off from the filament as separate structures and develop into oogonia. The 

 writer distinguishes these filaments as female. Other of the filaments in- 

 crease in size and their contents are evenly distributed throughout their 

 length. These strands branch irregularly and often give rise to forks which 

 may develop into antheridia. These are described as male filaments. 



13) Fertilisation occurs between the ripe antheridia and oogonia 

 giving rise to an oospore. The oospores appear either singly or joined in 

 twos and threes in the disorganised cellular tissue of the diseased patch. 

 Thej' are spherical, 20 ^ to 38 [x in diameter, with thick, smooth walls. 



14) The oospores can germinate as soon as they are formed. They 

 are in no sense resting spores to insure the survival of the fungus after 

 the winter, but true summer spores whose life has a very definite duration. 



