IOO Practical Plant Biology. 



thickened and loses its extensibility. While it is thus being 

 matured a second conidium is produced from the sterigma below 

 and pushes up the first. Repetition of this prdcess produces 

 a chain of conidia from the tip of each sterigma. The older 

 conidia forming the outer end of the chain have a blue coloration 

 in their walls which confers the characteristic blue colour on 

 the tips of the mature conidiophores and has caused this plant 

 to be called blue mould in contradistinction to Mucor, which 

 not having this colour, is called white mould. These older 

 conidia are very easily detached from the sterigmata and being 

 very minute (2^ in diameter) are readily borne on the feeblest 

 draughts of air and scattered far and wide. When they fall on 

 suitable substances they absorb water and their protoplasm, which 

 at first filled the cell-wall entirely, now becomes vacuolated. The 

 wall is thus distended and the conidium increases in size. Then 

 it forms a bulge and this grows out into a cylindrical cell. The 

 nucleus subdivides and so the cell becomes a ccenocyte, which 

 elongates, divides and so gives rise to a hypha. By branching 

 in the manner described above, the hyphae develop into a 

 mycelium and the latter ultimately produces conidiophores and 

 conidia, thus completing the life-history. The conidia are 

 single asexual cells and may be classed as spores according to 

 our definition. 



Reproduction by means of conidia occurs in the normal growth 

 of this fungus and is very prolific, but when it develops under 

 conditions in which the supply of oxygen is limited, Penicillium 

 resorts to another method the sexual one. 



This process is begun by the development on the mycelium 

 at numerous places of pairs of short branches, which we may 

 provisionally call gametangia. These gametangia are cylindrical, 

 their bases are close together and they coil round one another 

 spirally. Actual fusion of the protoplasm of the two gametangia 

 is believed to take place, but has not been observed. The 

 coiled gametangia now give rise to short branches called as- 

 cogenous hyphse which grow out in every direction. Meanwhile 

 the hyphse which support the gametangia also form numerous 

 branches which grow up over the ascogenous hyphae interlacing 

 in a confused tangle with one another and forming a compact 

 covering over them of several layers in thickness. The walls 

 of the outer layers become thickened and dark -yellow in colour 

 and form a hard, resistant rind to the mass which is now spherical 

 in form. After a pause in growth of several weeks the ascogenous 

 hyphae form a series of peculiar thick branches with hook-like 



