272 



NATURE 



[July 19, 1888 



leaves are stripped, the cambium suffers starvation to a 

 greater or less extent, depending on the intensity of its 

 competition with other tissues, &c. ; of course a starved 

 cambium will form less wood, and, it may be added, the 

 timber will be poorer. 



Again, even if the leaves are not stripped quickly from the 

 tree, but the effect of some external agent is to shorten their 

 period of activity ; or to occupy space, on or in them, and 

 so diminish the amount of leaf-surface exposed to the light 

 and air ; or to block up their stomata, the points of egress 

 and ingress for gases and water ; or to steal the contents 

 of the cells — contents which should normally be passed 

 on for the growth, &c, of other parts of the tree— in all or 

 any of these ways injury to the timber may accrue from 

 the action of the agent in question. Now there are 

 numbers of parasitic fungi which do all these things, and 

 when they obtain a hold on pure plantations or forests, 

 they may do immense injury before their presence is 

 detected by anyone not familiar with their appearance 

 and life-histories. 



The great difficulty to the practical forester who 

 attempts to deal with these "leaf diseases" is at least 

 twofold ; for not only are the leaves so numerous and so 

 out of reach that he can scarcely entertain the idea of 

 doing anything directly to them, but (and this is by no 

 means so clearly apprehended as it should be) they stay 

 on the tree but a short time as a rule, and when they fall 

 are a continual source of re-infection, because the spores of 

 the fungi are developed on them. It is a curious fact that 

 those fungi which are known to affect the leaves of forest- 

 trees nearly all belong to two highly-developed groups — 

 the Uredineae and the Ascomycetes — and the remarkable 

 biological adaptations which these parasites exhibit for 

 attacking or entering the leaves, passing through periods 

 of danger, and so on, are almost as various as they are 

 numerous. Some of them, such as the Erysiphece or 

 mildews on beeches, oaks, birches, ashes, &c, only form 

 small external patches on the leaves, and do little if any 

 harm where the leaf-crown is large and active ; others, 

 such as many of the very numerous Sphczriacece and 

 their allies, which form small dark-coloured flecks and 

 spots on leaves, may also be looked upon as taking only a 

 slight tax from the leaves. Even in these cases, however, 

 when the diseases become epidemic in certain wet seasons, 

 considerable damage may accrue, because two chief causes 

 (and many minor ones) are co-operating to favour the 

 fungus in the struggle for existence : in the first place, a 

 continuously wet summer means loss of sunlight and 

 diminished transpiration, &c, to the leaves, and so they 

 form smaller quantities of food materials ; and secondly, 

 the damp in the atmosphere and leaves favours the fungi, 

 and so they destroy and occupy larger areas of leaf 

 surface. 



It should be mentioned here, by the way, that all leaves 

 of all trees are apt to have fungi on them in a wet summer, 

 but many of these are only spreading their mycelia in all 

 directions over the epidermis, in preparation, as it were, for 

 the fall of the leaf : they are saprophytes which feed on 

 the dead fallen leaves, but cannot enter into them while yet 

 alive. In some cases, however, this preparation for the fall is 

 strikingly suggestive of adaptation towards becoming para- 

 sites. 1 will quote one instance only in illustration of this. 

 On the leaves of certain trees in Ceylon, there was always 

 to be found in the rainy season the much-branched 

 mycelium of a minute Sphceria : this formed enormous 

 numbers of branches, which, on the older leaves, were 

 found to stop short over the stomata, and to form 

 eventually a four-celled spore-like body just blocking up 

 each stoma on which it rested. So long as the leaf 

 remained living on the tree, nothing further occurred ; but 

 wherever a part of the leaf died, or when the leaf fell 

 moribund on the ground, these spore-like bodies at once 

 began to send hyphae into the dying tissue, and thus 

 obtained an early place in the struggle for existence 



among the saprophytes which finished the destruction of 

 the cells and tissues of the leaf. 



There is another group of fungi, the Capnodiece, which 

 form sooty black patches on the leaves, and which are 

 very apt to increase to a dangerous extent on leaves in 

 damp shady situations : these have no connection with 

 the well-known black patches of Rhytisma from which the 

 leaves of our maples are rarely free. This last fungus is a 

 true parasite, its mycelium penetrates into the leaf tissues, 

 and forms large black patches, in and near which the cells 

 of the leaf either live for the benefit of the fungus alone, 

 or entirely succumb to its ravages : after the leaf has 

 fallen, the fungus forms its spores. Nevertheless, although 

 we have gone a step further in destructiveness, foresters 

 deny that much harm is done to the trees— no doubt 

 because the foliage of the maples is so very abundant. 

 Willows, pines, and firs suffer from allied forms of fungi. 



But it is among the group of the Uredinecc or rusts 

 that we find the most extraordinary cases of parasitism, 

 and since some of these exhibit the most highly developed 

 and complex adaptations known to us, I propose to select 

 one of them as the type of these so-called " leaf diseases." 

 This form is Coleosporium Senecionis {Peridermium Pint), 

 rendered classical by the researches of several excellent 

 botanists. 



It is true, Coleosporium Senecionis is not in some 

 respects the most dangerous of these fungi — or, rather, it 

 has not hitherto been found to be so— but in view of the 

 acknowledged fact that foresters have not as yet been 

 able to devise practical measures against the ravages of 

 these numerous rust-fungi, and since we are as yet very 

 ignorant of the details of the biology of most of them, it 

 seems advisable to choose for illustration a form which 

 shows in a distinct manner the complexities of the subject, 

 so that those interested may see in whit directions 

 biologists may look for new results. That the story of 

 this fungus is both complicated and of great biological 

 interest will be sufficiently evident from the mere recital 

 of what we know concerning it. 



H. Marshall Ward. 



{To be continued) 



MIC HELL'S PROBLEM. 



"C'OR the last two hundred years the attention of logi- 

 -*■ cians and mathematicians has been directed to the 

 inverse principles of the theory of probability, in which 

 we reason from known events to possible causes. Two 

 different methods of calculation are in use, which give 

 approximately the same results. According to the cele- 

 brated theorem of James Bernoulli, " If a sufficiently 

 large number of trials is made, the ratio of the favourable 

 to the unfavourable events will not differ from the ratio 

 of their respective probabilities beyond a certain limit in 

 excess or defect, and the probability of keeping within 

 these limits, however small, can be made as near certainty 

 as we please by taking a sufficiently large number o 

 trials." The inverse use of this theorem is much mon 

 important and much more liable to objection and diffi 

 culties than the direct use. In the words of De Morgan 

 " When an event has happened, and may have ha 

 pened in two or three different ways, that way whic 

 is most likely to bring about the event, is most likely to 

 have been the cause." 



The second principle, due to Bayes, is thus given by 

 De Morgan, " Knowing the probability of a compound 

 event, and that of one of its components, we find the 

 probability of the other by dividing the first by the 

 second." 



These principles have been accepted by the great 

 majority of thinkers, and freely used by Laplace, Poisson, 

 Herschel, and De Morgan. Stanley Jevons (" Principles of 

 Science") gives a luminous account of the value of the 



I 



