14. 



Letters on the Diseases of JPlants. 



As for curing rust by treating the seed, the idea is ridiculous. It would 

 be just as reasonable to expect to prevent measles among mankind by 

 soaking babies in some sort of pickle. Eust is a disease that attacks wheat 

 after it is above ground. So far as is known the seed is almost never 

 attacked. In this respect rust differs radically from bunt. Bunt does 

 attack the seed — the seed particularly. This is the reason that various 

 solutions applied to the seed will prevent bunt. 



Fig. 13.— Spray of flax attacked by rust, natural size. The rust pustules are shown on the 

 leaves and branches. The rust, here provisionally named Melamipsora lini, Per§., may 

 be described as follows : — 



Uredo Stage. — The nearly spherical orange-coloured uredo- 

 spores form on the stem, leaves, sepals, and capsules of the 

 host plant golden yellow round to oblong, or (on the stem) even 

 linear pulverulent sori, which vary from one to five millimetres 

 in length, the larger sori being undoubtedly due to the conflu- 

 ence of several smaller ones. WhUe yet young the sori have 

 the appearance of small blisters. When the uredosori are 

 mature, the leaves of the host plant are sometimes Completely 

 obscured by the powdery mass of uredospores present on its 

 surface. A leaf but little more than an inch in length may bear 

 as many as 200 sori, and even the average number of sori on 

 a leaf often exceeds 100. The uredospores are borne on stalks 

 among numerous capitate paraphyses (see Fig. 14.) These para- 

 physes occur throughout the sorus, but are less numerous near 

 the centre. Their nearly transparent, smooth, spherical to clavate 

 heads are somewhat larger than the spores, and are borne on 

 stalks longer than those of the spores. The marginal portion of 

 the sorus is composed completely of jjaraphyses, a fact easily 

 demonstrated by means of sections through the sorus, or by 

 examination from above with a medium power. Each sorus is 

 enclosed in a pseudoperidium composed of a single layer of 

 spherical, or, rather, polygonal cells, whose diameter is about 

 one-third as great as the transverse diameter of one of the epi- 

 dermal cells of the leaf of the host-plant. This pseudoperidium 

 is often visible to the unaided eye as a somewhat lacerated mem- 

 brane bordering the sorus. It has been mistaken for the 

 rviptured epidermis of the host-plant. It is easily removed for 

 examination, or its existence and structure may be demonstrated 

 by cross-sections of the sorus. A similar pseudoperidium is 

 known to exist in Mclatiipsora 'poindhia, and in a few other 

 cases. The origin and development of these pseudoperidia of 

 the lu-edospore sori have not yet been sufficiently investigated. 

 Such well developed and persistent enveloping membranes are 

 well-known characteristics of the jEcidium stage of numerous 

 rusts, and the appearance in the lu-edo stage is another morpho- 

 logical evidence of the genetic connection between the old form- 

 genera jEcidimti and Urcdo. The finely echinulose uredospores 

 germinate readily, and when doing so give evidence of the 

 in-esence of at least three or four germ spores. They tlirow 

 out about two hyphse, only one of which is likely to grow 

 vigorously. This one is of irregular diameter, and often, though 

 not always, gives rise to numerous finger-shaped branches ; the 

 total leng-th of the system thus produced often exceeding ten 

 times the length of the diameter of the spore. The germin- 

 ating spores measure 23 to 29 fi in diameter, being nearly 

 spherical. 



Puccinia Stage. — The sessile, one-celled, cylindroid, or per- 

 haps it would be better to say prismoidal, brownish teleuto- 

 spores are closely packed in dark brown sori, principally on the 

 stem of the host-plant, and for some time remain covered by its 

 epidermis. They are from four to five times as long as broad 

 (11-15 X 57-73 f^), being straight near the centre of the sorus, 

 and slightly curved near the margin. At the free extremity of 

 the teleutospore, where the wall is thicker and darker brown, 

 is fovind a single germ-jjore. 



The above-described rust may not be M. lini, Pers. It is 

 , common in New South Wales on Linum viarginale, a native flax, 



and has been found on experimental crops of linseed— that is, Lin um usitatissimum. The linseed 

 is much injured by the rust. According to Barclay, this fvmgus is apparently extremely common 

 over large areas of the plains in India. The specimen forwarded by Mr. Clout, of Eosemount, 

 Brungle, is a plant grown from Indian seed. Mr. Clout said New Zealand seed gave sound plants. 



Various inquiries about the rust on flax lead me to point out that this rust 

 differs materially from that attacking wheat, however much alike the two 

 may appear to the unaided eye, and that it never attacks wheat. Nor does 

 the wheat-rust ever attack flax. 



