60 THE FUNGI WHICH CAUSE PLANT DISEASE 



protoplasm is therefore continuous througliout the whole plant 

 body and may be regarded as constituting one cell though it may 

 be of great extent and bear very numerous nuclei. Such multi- 

 nucleate cells, coenocytes, may be regarded as cell complexes with 

 the walls omitted. 



In one comparatively small order, the Chytridiales, there is 

 often no filamentous mycelium and the vegetative body consists 

 merely of a globular, irregularly spherical or amoeboid cell. Such 

 forms are thought by some mycologists to be degenerate, to have 

 in remote time possessed a mycelium which has been lost owing to 

 the present simple mode of life of the fungus, the needs of which 

 no longer call for a filamentous body, while others ^ find here 

 primitive forms of Phycomycetes, and trace their phylogenetic 

 connection with the higher orders of the class. 

 Reproduction. 



Vegetative. Most mycelia, if cut in bits and placed in suitable 

 environment, continue to grow, soon equaling the parent mycelium 

 in size if abundant nourishment obtains. Bits 

 of diseased tissue, bearing mycehum, thus con- 

 stitute ready means of multiplication and dis- 

 persal. 



Asexual Spores. A spore is a special cell set 

 aside to reproduce the plant. An asexual spore 

 is a spore not produced by a sexual process. 

 Manifold forms of asexual spores exist among 

 the fungi. In some of the simplest cases, bud- 

 like out-growths (gemmae) appear on the myce- 

 lium; or portions of the mycelium itself are cut 

 off by partitions and the protoplasm inside 

 gathers into a mass and protects itself by a 

 firmer wall than that of the mycelium, chla- 

 mydospores. In other cases special branches, 

 ^'of^^cT^'diu™ ^yP^*' ^""e set apart for the purpose of bearing 

 Oidium. After spores. If the spores are cut off from the tip of 

 the branch they are known as conidia or conidio- 

 spores, and the branch bearing them is a conidiophore. Conidia 

 may be borne singly or in false clusters caused by the youngest 

 pushing the older conidia aside; frequently they are produced in 



