264 



THE TROPICAL AGRICULTURIST. 



[September i, 1882. 



pjTiform vesicle appears very early, and receives the whole 

 of the conteuts. And now the meaning of the vesicular 

 swelling above described becomes clearer, for it is usually 

 formed over the orifice of a stoma and sends its processes 

 through this into the intercellular spaces of the leaf 

 (PI. II, fig. 18). This is, in fact, the act of " infection." 

 The Uredospore on germinating produces a simple mycelium 

 (the germinal tube), which grows rapidly at the expense 

 of the reserve material in, the spore, and is only capable 

 of further progress on reaching the interior of the leaf 

 in this manner. 



The commencement of this furtlier development is well 

 shown iu figs, 10 and 20. That the internal mycelium 

 is simply an extension of the germinal tube is proved 

 by such example as fiig. 21 : the spore germiuating close 

 to the orifice of a stoma, has sent its tube directly thi-ough 

 into the leaf without forming the preliminary ve.sicle. 

 These and many similar preparations were obtained by 

 sowing spores on living leaves kept damp in glass cells, 

 and cutting vertical sections at periods varying from 24 

 to 100 hours afterwards. 



I carried still further the proof of the fact that the 

 internal mycelium is but an extension in the leaf of the 

 germinal tube by sowing .spores on the up2>ef sm-faee of 

 leaves at places from which the epidermis had been re- 

 moved; the result was a rapid growth of the germinal 

 tube directly into the tissues, pushing its way between 

 the palisade cells as it advanced (figs. 23 and 23). Here 

 again no vesicle was formed. All attempts to infect by 

 sowing on the uninjured upper surface have failed ; the 

 spores germinate, tubes and vesicular swellings form as 

 on glass, but the whole soon .shrivels and dies. 



The mycelium mthiu the leaf, then, the action and 

 extension of which corresponds to the yeUow discoloration 

 seen externally, is clearly but a continuation of the germ- 

 inal tube sent forth from the Uredospore, and which 

 enters a stoma as described. Once established in the 

 lacuuie of the leaf this soon branches, chiefly at first 

 in the plane of the leaf, and feeding ui>on the products 

 of the cells of its host, produces the injury.* At first 

 the young mycelial tubes are very delicate, flUed with 

 fine-grained -protoplasm, and somewhat stumpy ; they soon 

 become vacuolated, and more coar.sely granular, and send 

 out tufts of short, thick branches towards the cells 

 bounding the intercellular spaces, while here and there 

 longer " leaders " run out between the cells in various 

 directions. 



The main features of the internal mycelium thus pro- 

 duced are typically as follows. Its ramifications are con- 

 fined to the intercellular spaces (fig. 24), except that at 

 numerous points here and there very slender processes 

 pierce the cell-wails to form haustoria. The mode of 

 branching is extremely irregular, and influenced by the 

 arrangement of the cells between which the branches 

 run ; the rate of growth, depending on several circum- 

 stances, also aflfects the length of the branches. 



Transverse septa occur here and there, often seijarated 

 by long intervals (fig. 28), especially at the peripheral 

 parts. The diameter of a mycelial thi'ead is about the 

 same as that of the germinal tube, and, like that, 

 generally preserves its uniform calibre throughout. In 

 some cases the similarity between mycelium and germinal 

 tube is rendered more striking from the contents of the 

 former being also coloured orange-red ; as a rule, how- 

 ever, the protoplasm of the internal mycelium is colour- 

 less. Vacuoles, granules, and oily drops occur commonly 

 in fresh preparations (fig. 27), but in some cases the hiauch 

 is filled with a dense, homogeneous protoplasm, shining 

 mth a grey, pearly lu.stre. (fig. 26). The "coral-Uke" habit 

 of the tufted, short-branched form is well seen in figs. 

 25 aud 2S. 



The haitstorium is a somewhat remarkable structure. It 

 consists of a stiff, long neck, piercing the cell wall vertically 

 from a branch of the mycelium (figs. 25, 29, 30) ; the 

 distal end is expanded into an ovoid or pyriform body, 



* As further evidence, I may remark the success of in- 

 fection experiments based on these observations. In one 

 instance. 1 made sixteen separate sowings on healthy leaves 

 of as many plants: in JifteeA cases the "disease spot" 

 appeared vherc the sowing was made, and nowhere else oil 

 tlie plant . 



suspended free in the cell cavity, and containing usually 

 one or two brilliant granules surrounded by a cloudy matrix. 

 In older specimens a distinct wall is evident. Spreading 

 in all directions from the point of entry, the myceUal 

 branches become applied to the exterior of the cells, and 

 feed upon their contents by means of these haustoria, 

 until a stage is reached when the well-nourished vegetative 

 structure commences to form the .spores, which appear 

 externally as " rust." 



This process is begun by a tuft of branches collecting 

 in a lacuna, and growing towards the orifice of the stoma, 

 close to which their apices remain in contact for a short 

 time ; the tightly-packed bundle then forces itself into 

 the orifice, and pushes the common apex through to the 

 exterior (figs. 32 and 33), where the spores are formed 

 by budding. 



The first spores arise as follows : — The spore-bearing 

 branches, formed as above described, are filled with fine- 

 grained grey protoplasm (fig. 33), aud on reaching the 

 exterior the apex of each expands into an ovoid .sac (PI. 

 Ill, fig. 34), in which the protoplasm accumulates. A 

 succession of other similar sacs arise by budding from 

 the parts below this, and thus a tuft of young spores is 

 formed (fig. 35 and fig. 40 n, h, c). Each of these spores 

 arises by the protrusion of an ovoid sac, remaining narrow 

 below, and becoming constricted off at the neck, where 

 a septum also is formed separating the young spore from 

 the spore-bearing branch (fig. 40). A small pedicel or 

 stalk is usually formed, but the spore is readily detached 

 from tliis, and it is rarely seen on adult specimens, though 

 the circular place of attachment may frequently be detected. 



Each spore consists at first of a simple, smooth, thin- 

 walled sac, filled with fine-grained protoplasm, in which 

 a uucleus-like body may be frequently observed (fig. 40, 

 c and e). At a very early stage the existence of an 

 endospore can be proved, but the exposure remains smooth 

 and thin for some time. As the regular ovoid shape of 

 the spore becomes altered by lateral and other pressures, 

 the thickening exospore develops the solid papillce above, 

 and the contents begin to assume the orange-red tinge. 



As the spore-bearing branches (each of which forms 

 spores as above at first) pass through the orifice of the 

 stoma the,y are crowded together into a neck ; below and 

 above this constriction they expand again. As age advances, 

 however, they are found to become coalesced into a kind 

 of i)seudo-parenchyma, and the later-formed spores arise 

 from the sides and top of a compound body (figs. 39 and 

 40 rf), produced by their union. This structure presents 

 the form of an oval boss, with its lower side attached 

 by a neck, which passes through the stoma to the mycelium 

 within the leaf ; its .sloping sides are covered with crowds 

 of short, stumpy processes (fig. 41), the remaijis of ped- 

 icels from which spores have fallen. The fusion of mycelial 

 elements may even extend to the internal position close to 

 the neck, and possibly the " dark body " figured by Abbay* 

 is this structure, into which air had penetrated. 



Viewed from above, the various stages of spore form- 

 ation are easily discovered. The story is the same. A few 

 ovoid young spores arise (fig. 41 a, Ii), and additional ones 

 bud off from near their bases (f), until a rosette-like cluster 

 is formed (e). The appearance of the old spore-bearing head, 

 formed by the fusion of the spore-bearing branches, is 

 figured at fig. 41 ,/". "When the " disease spot " has ceased 

 to spread, and all, or nearly all, the spore-bearing heads 

 have become as advanced as this, the second form of 

 spore is produced. 



This Tftei'tospore is a very remarkable body, and it is 

 only iuteuded here to describe its morphological character- 

 istics. It isatfirst indistinguishable from the yonngZh'edospore, 

 but, iustead of developing into the reniform papdiate 

 strticture, it remains somewhat smaller, quite smooth, and 

 expands into a subglobular shape. "When fully formed it 

 is napiform, aud situated on a short stalk (fig, 44) attached 

 to the .spore-bearing head already described (figs. 38 and 39). 

 An endospore is early distinguishable, and the granular 

 protoplasmic contents soon become coloured bright orange 

 red. 



Very soon after its complete formation the central portion 

 of the free, slightly flattened end of the oblate spheroid 

 protrudes as a rounded, blunt, boss-like eminence; this 



* Loc. cit., p. 177, Plate 13, fig. 4, &c. 



