Pethybridgh: and Lafferty — Dry-Rot of the Potato Tuber. 205 



and Wollenweber had already suggested) which gives rise to the foot-like 

 shape sometimes exhibited by the basal-cells of the conidia. 1 



The conidia produced on the pustules are slightly sickle-shaped and 

 rather bluntly pointed at both ends. In general shape they somewhat 

 resemble that of a pea-pod; and, just as the latter is enclosed at its base by 

 the persistent calyx of the flower, so the base of the conidium rests within 

 the sheath, as described above. 



Typical conidia are shown in fig. 2, Plate VII. When produced in great 

 quantities, under moist conditions, the conidia form rather slimy masses on 

 the surfaces of the pustules. The conidia on the pustules are usually from 

 0- to 3-septate ; i.e. from 1- to 4-celled ; but the 3-septate forms always 

 predominate. (In cultures 5- and 6-celled conidia are also produced.) In 

 size they vary considerably, according to the conditions of moisture under 

 which the tuber is decaying. The averages (in microns) for the various forms 

 are as follows : — 



0-sepitate. 1-septate. 2-septate. 'A-septate. 



14 x 3-7. 19 x 4-1. 21 x 4-3. 27 x 4-8. 



In healthy artificial cultures the conidia produced are generally s'omewhat 

 larger than those found on the pustules. 



The chlamydospores, which occasionally occur on the pustules, are more 

 or less spherical, possess somewhat thick, smooth walls, and are blue in 

 colour. They are produced singly, in pairs, in chains, or in irregular groups. 

 Their average diameter is 8 to 9 /x. Chlamydospores are shown in figs. 4 and 

 5, Plate VII. 



1 It may, perhaps, be pointed out that the explanation of the origin of this "foot" 

 structure of the basal cell of the conidium given by Wilcox, Link and Pool (loc. cit. 

 p. 23) does not appear to be correct. The figures on which the explanation given by 

 these authors is based are of hyphae, and not of conidia. If their explanation is correct, 

 both the apical and the basal cells of a conidium should show the "foot" ; but we have 

 not found this to be the case. 



The sterigma-sheath is clearly illustrated by Sherbakoff (loc. cit. p. 129, fig. J) for 

 F. cuneiforme, but its significance is not alluded to. We have found that the foot-like 

 appearance of the basal cell is most strongly developed in old and nearly-exhausted 

 cultures, in which conidia are produced only slowly. In such cases the extremity of the 

 basal cell must remain confined within this sheath for a considerable time, while the 

 part immediately beyond it is free to expand. Since the conidium seldom sits symme- 

 trically in this sheath, the small heel-like swelling usually occurs only on one side, and 

 thus gives the foot-like appearance to the basal cell. 



Seeing that the shape of this basal cell is, therefore, dependent on conditions of 

 growth or culture, too much stress must not be laid on it as a morphological character 

 for any particular species. In F. cosruleum, it is not usually a very marked character, 

 and frequently it is entirely absent. 



