Sporidiola or Promycehal Spores. 27 



CHAPTER IX. 



SPORIDIOLA' OR PROMYCELIAL SPORES. 



When the teleutospore germinates, whether at once or afiter a period of 

 rest, the endospore is protruded through the germ-pore as a germ-tube, 

 and the contained protoplasm passes into it. This germ-tube does not 

 behave like that of the aecidiospore or uredospore, and elongate and branch 

 indefinitely, but it soon ceases to grow in length, and terminates blindly. 

 Hence it has received the special name of promycelium, because it directly 

 produces its spores. The promycelium is soon divided by septa, generally 

 stated as formed from above downwards, but my own observations in 

 regard to Puccinia malvacearum show that the median septum is first 

 formed, dividing the promycelium into two, and then each of these 

 subdivides again into two, making in all four cells, from each of 

 which there is a short lateral protuberance which dilates at the end, and 

 becomes a promycelial spore or sporidiolum. The two upper cells, as a 

 rule, produce their sporidiola first, then the next, and lastly the lowest ; 

 but occasionally .the two median cells start first. The name sporidium is 

 often applied to this spore, but since it is already used as equivalent to 

 ascospore, the present name has been proposed by Saccardo. 



The sporidiola are easily detached, and, provided with moisture, they 

 can germinate at once, and on the surface of a living leaf the germ-tube 

 can pierce the epidermis, and, growing and branching in the interior, pro- 

 duces a mycelium similar to that derived from the aecidiospore or uredo- 

 spore. But in the case of grasses the walls of the epidermis often contain 

 much silica, and this may be one of the reasons why aecidia are so scarce 

 in that family. Blackman * has shown that there is considerable variation 

 in the length of the promycelium, according to the conditions of growth. 

 Teleutospores, germinating in moist air, produce very short promycelia, and 

 form sporidiola almost immediately, while those germinating in drop cul- 

 tures, with their germ-tube submerged, grew to a length only limited by 

 the reserve material, but no sporidiola we're formed. On reaching the air, 

 however, their formation took place. The free air necessary to their for- 

 mation is correlated with their distribution by the wind. 



If the teleutospore be regarded as the final stage of the cycle, then the 

 sporidiolum will be the starting point, and from that there may proceed in 

 regular succession the various spore-forms already enumerated, viz., sper- 

 matia, aecidio, uredo, and teleutospores, back to sporidiola again. All these 

 may occur in the same species, and the general course of development is 

 briefly as follows: From the teleutospore in the spring, sporidiola are 

 formed which develop on a suitable host-plant a mycelium, from which 

 usually on the upper side of the leaf spermogonia are developed, and either 

 on the same side, but generally on the opposite side, aecidia are soon after- 

 wards produced. Infection by the aecidiospores produces the uredo-stage, 

 and these spore-forms often reproduce themselves. The uredo is specially 

 adapted for ithe rapid spread of the fungus, since from the time of infection 

 up to the formation of new uredospores, only eight to ten days may 

 elapse. Finally, along with the uredo, or in special layers, teleutospores 

 arise which, on germination, produce again the promycelia and sporidiola, 

 and thus complete the course of development. This is the typical mode in 

 which the alternation of spore-forms occurs in many species belonging^ to 

 different genera, but there is often variation in the order, or even omission 

 of some of the stages. So constantly are aecidial or uredo stages associated 



