SCIENCE-GOSSIP. 



237 



BIOLOGICAL JOTTINGS. 



Part II. — Parasites. 



By Rudolf Beer, F.L.S. 



TN a former "jotting" {ante p. 173,) I pointed 

 out the uncertainties which surround our 

 ideas of the organic cell. In writing this second 

 paper my thoughts were turned to a subject which 

 was in everyone's mouth, and about the meaning 

 of which there could be no dispute. Parasitism 

 was the chosen theme. But, alas for my hopes ! 

 At the very outset, when I attempted to write 

 down a definition, I found myself beset with more 

 trouble and more confusion than even that 

 chameleon term "cell" brought with it. After 

 much reflection and much hesitation, Mr. Michael 

 hazards the definition of an animal parasite as 

 " a creature which, at the time spoken of, is 

 residing in a permanent or temporary manner in 

 or upon another living creature, and is existing at 

 the expense of or by the assistance of the host." 



Even this statement, which is perhaps as 

 satisfactory as any can be, leads to strange issues. 

 The sheep, living upon and at the expense of the 

 grass of the meadow, is, from this point of view, 

 undoubtedly a parasite. Moreover, the doubt arises 

 that perhaps when we are enjoying a leg of mutton 

 we also are parasites ! We will pass, however, 

 from such painful definitions to instances of what 

 are commonly regarded as parasites. An example 

 of the most virulent type of parasite is furnished 

 by certain members of that all too familiar race, 

 the bacteria. Another illustration is given by 

 the Uredineae, a class of fungi which play the 

 same baneful part among the higher plants as 

 bacteria do among human beings. It may not be 

 unprofitable to look a little more closely at these 

 fungi. A remarkable feature of these plants is 

 the fastidiousness with which they select a host. 

 Not only do they demand a particular species on 

 which to grow, but many of them spend the 

 different phases of their existence on distinct and 

 special hosts. To gain some idea of their curious 

 habits, we will glance at the life cycle of a common 

 and representative member of the class. 



The mildew of wheat will be familiar to every- 

 one. Man's acquaintance with this pest dates 

 back to very remote antiquity ; in the middle 

 ages Wycliffe mentions it, and Shakespeare 

 speaks of the foul fiend who " mildews the white 

 wheat." That some connection exists between 

 this wheat disease and" the barberry plant was 

 recognized by farmers long before its fungoidal 

 nature was understood. In fact, in 1760 an Act 

 was passed in Massachusetts to effect the exter- 

 mination of all barberry bushes, owing to the 

 spread of wheat mildew. 



K 



The true nature of the disease of wheat and of 

 the influence of the barberry plant upon it was 

 first explained in 1818 by the patient and unob- 

 trusive work of a Danish schoolmaster, Schoeler 

 by name. Finally the master hand of De Bary 

 cleared up the remaining difficulties and set the 

 whole story before us with limpid clearness from 

 beginning to end. 



Thanks to this brilliant work, we now know that 

 the barberry itself is afflicted by a disease caused 

 by a parasitic fungus of the class of Uredinere. 

 The spores produced by this fungus, strange to 

 say, will not develop upon the barberry bush, but 

 only upon wheat, and here they grow into a form 

 quite different from their parent. Further, the 

 spores of this second phase germinate upon fresh 

 wheat plants to produce a different adult form 

 from either of those which have gone before. From 

 the resting spores produced by this third form a 

 different generation is originated, the spores of 

 which, falling upon a barberry, reproduce the 

 stage with which we commenced. During the 

 sojourn of this parasite upon the wheat the mildew 

 of the cereal is caused. We see then that there 

 are four stages in the existence of this organism ; 

 one of these is spent on the barberry, two on the 

 wheat, and one is independent of a host plant. 

 Truly, we may admit, this is a strange and 

 complex life-history. It is a story too which is 

 not peculiar to wheat blight, but one that is shared 

 in a greater or less degree by all the members of 

 that large group of parasites, the Uredineas. For 

 a long time each stage was looked on as a separate 

 individual, but now the connections of the different 

 forms are being gradually worked and understood. 

 The name of " saprophyte " is given to those 

 parasites which, although not attacking living 

 organisms, are yet dependent upon their dead 

 remains for sustenance. An example is furnished 

 by Xylaria hyponylon, the candle-snuft" fungus which 

 grows on dead stumps of wood. 



Besides these two degrees of parasitism connected 

 with destruction and decay, there is also a third 

 form in which nothing but good and advantage 

 accrues both to host and parasite. I refer to the 

 condition of symbiosis, which may be defined as a 

 state of partnership entered into by two organisms 

 for their mutual benefit, or, at any rate, if for the 

 direct good of one alone, without the harm of the 

 other. A very excellent example is furnished by 

 the nodules on the roots of the leguminous plants. 

 These tuberosities are crowded with micro- 

 organisms which " fix " the free nitrogen of the air 



