Vegetative Organs. 



CHAPTER II. 



VEGETATIVE ORGANS MYCELIUM. 



The vegetative portion of many fungi is very inconspicuous as com- 

 pared with the reproductive, but its importance is not to be measured by 

 its size or extent, rather by the part it plays in the life of the organism ; 

 and since it is the foundation of the whole, it' is worthy of the most careful 

 study. 



Of late years, however, this part has come into special prominence, 

 particularly in the case of the cereal rusts, for it has been asserted that 

 it is not always by external infection that the rust begins its career in the 

 growing plant, but that in some cases it originates from within, and this 

 theory will engage our attention later on. Meanwhile this is referred to 

 to show that the key to the propagation of the rusts from year to year may 

 be not only on the surface, among the special reproductive bodies which 

 spread it throughout the growing season, but also in the interior among the 

 cells where the first beginnings of its life may appear. 



Among the recent investigations on the mycelium, there are two which 

 *tand out on account of their completeness, owing to the use of the most 

 modern histological methods the one by Professor Marshall Ward 9 on the 

 Histology of Uredo dispersa Eriks, and the Mycoplasm hypothesis, and the 

 other by Professor Eriksson 14> 15 on Das vegetative Leben der Getretderost- 

 pihe [The Vegetative Life of the Cereal Rusts]. The study of an indivi- 

 dual case will prepare us for the more general examination of the 

 mycelium throughout the rusts, and we will begin with that of Puccinia 

 dispersa Eriks., or, strictly speaking, P. bromina Eriks., which Ward has 

 so thoroughly dealt with and illustrated with such admirable clearness. 



Starting with the germination of the uredospore on the surface of the 

 leaf, which usually occurs within twenty-four hours, we find that the young 

 germ-tube grows rapidly, and that the nucleus of the spore passes into it ; 

 sometimes, however, two or more nuclei may appear in it. The tip of the 

 tube begins to swell over a breathing pore or stoma into a thin vesicle, and 

 the contents derived from the spore accumulate here. 



This external vesicle or appressorium, as it is called, is the first stage 



in inoculation from the outside, for a thin process is passed through the 

 opening of the stoma, and swells inside into another vesicle. The proto- 

 plasmic contents are transferred from the external to the internal vesicle, 

 and so the future growth takes place among the tissues of the leaf. 



At one or more points this inmer swelling forms a delicate tube, into 

 which the protoplasm is again transferred, and its nucleus soon divides. 

 This is the first-formed hypha, and the foundation of the vegetative system. 

 It soon branches and develops cross partitions or septa, and extends rapidly 

 among the cells of the host-plant to form the mycelium. Even at an early 

 stage, when the primary hypha is still unbranched and unseptate, suckers 

 or liaiistoria may be formed to provide a large imbibing surface for the 

 fungus. The haustorium begins as a small delicate process or projection 

 from the hypha, and this pierces the cell-wall and swells up into a minute 

 spherical head, which is provided with a nucleus. Shortly after entering 

 the cell this head takes on an irregular growth, and may assume a variety 

 of shapes. 



The mycelium now becomes denser towards the surface, and prepares 

 for the production of the reproductive bodies or spores. This constitutes 

 the history of the mycelium from the time it starts as a germ-tube until 

 it reaches its full development. 



