JO 



Reproductive Organs. 



relation or the effect of atmospheric moisture upon the spores or mycelium 

 it has been shown that dew is absolutely necessary for infection, and of 

 more importance .than rain, and, in fact, that without dew there can be no 

 infection. Further, that the effects of atmospheric dryness not only 

 influence spore germination, but also spore production, and the aecidial .and 

 uredo stages art checked thereby, but if the mycelium is not completely 

 destroyed, the teleuto stage may appear independently of conditions u 

 favorable to the others. The indirect relation of water or the effect of soil 

 moisture, which acts upon the parasite through ifts host, has also been 

 studied and shown to be of great importance. An abundance or sojl 

 moisture at the growing season, in the case of asparagus rust m California, 

 is stated to increase the vigour and vitality of the host, and retard the 

 development of the fungus. That the growth of the fungus is retarded 

 by increased vitality of the host scarcely agrees with Marshall Ward s- 

 conclusions in regard to brome rust (Chap. XVIII.)- That the weather 

 exercises a commanding influence in hastening the development ? of rust in 

 wheat is abundantly shown by its rapid spread in hot " muggy weather, 

 especially when accompanied by heavy dews. Irrigation before ploughing 

 gives the wheat a good start, but if applied in the spring it makes the wheat 

 soft and very liable to rust, a case in which great vigor of growth of the 

 host is followed by a strong development of the rust. 



While the ordinary mode of infection is as stated above, exceptions may 

 occur. Thus, according to De Bary, the germ-tube of the sporidiolum of 

 Puccinia dianthi (P. arenariae} may enter by the stomata, and Bolley 4 

 gives illustrations to show that the germ-tube of the uredospores of P. 

 triticina may enter direct through the epidermal cells as well as by the 

 stomata. 



SUPPRESSION OR OMISSION OF SPORE-FORMS. 



As already pointed out in the Introduction, the complete cycle of develop- 

 ment follows an invariable order. The germinating teleutospore gives rise to 

 sporidiola, from the mycelium of which arises the spermogonia and aecidia ; 

 later the uredospores appear, and finally the teleutospores complete the 

 cycle. 



The aecidio, uredo, and teleutospore forms are represented respectively 

 for convenience by the Roman numerals I. II., III., and if the sporidioia 

 arising from the teleutospore are designated by IV., and spermogonia 

 by a cypher, then it will be comparatively easy to represent the different 

 stages 'diagramatically. A complete cycle of development will be repre- 

 sented thus O., I., II., III., IV. 



The teleutospore with its sporidiola is believed to be a constant factor 

 in the cycle, although .there are various rusts in which it has not yet been 

 found, so that I. or II., or both, may be omitted or suppressed, as well as O, 

 \\hirh, however, is rarely absent. The germinating sporidiolum produces a 

 myo'lium which may in turn give rise to spermogonia and aecidia. These may 

 either occur on the same species of host-plant as that which Dears the teleuto- 

 spore, and the rust is thus said to be autoecious, or they may occur on a 

 totally different plant when the rust is said to be heteroecious/The aecidio- 

 spore on germinating may skip the uredo stage, and directly produce the 

 teleutospore, and thus the cycle is shortened. There are numerous examples 

 of this in Australian forms, among which may be mentioned the rust on the 

 marigold (P. calendulae), and that on the daisy (P. distincta). 



This variation in the alternation of spore-farms has been used by 

 Schroeter as a basis of grouping, and although these biological sections by 

 no means indicate close affinity, yet they conveniently group together forms 

 \\hirh luuv similar associated stages. Taking the genus Puccinia as an 



