Jl-NE. 1904.] 



KNOWLEDGE c^ vSCIENTIEIC NEWS. 



115 



chlorophyll appai.iUi>. the loss of the power of food- 

 construction, and a consequent degradation of structure, 

 always found accompanying such impotence. 



The development of the parasitic habit has not taken 

 place among one group of plants alone, and the parasitic 

 plants are not therefore connected with one another by 

 any ties of descent or inheritance. Parasitism has been 

 acquired by simple and by complex plants, and ap- 

 parently more than one chain of circumstances has k^d up 

 to it. 



Among the simpler families wc find the great class of 

 bacteria, or germs, in many cases parasitic, liiough others 

 live on dead organic matter. Such as the latter are 

 designated sapropliytis, and in many cases they mark a 

 halfway house to parasitism. A group of somewhat 

 higher type is aflorded by the fungi or moulds, which, like 

 the former, include both saproph3tic and parasitic 

 forms. 



From their structure these parasitic fungi are closely 

 allied to the filamentous alga- or sea-weeds, from whicli 

 it is clear that they have been deri\ed. There are many 

 families of these aquatic organisms, distinguished from 

 each other by their peculiar methods of reproduction. 

 There are corresponding groups of fungi, and from com- 

 parison of them there can be little doubt that the fungi 

 have been developed from the alga^ which they resemble. 



The process of their development, though based upon 

 existing forms of both, can be fairly satisfactorily traced. 

 We have, however, only prohability to point to, for we have 

 not very satisfactory transitional forms. The main differ- 

 ence between the two is the presence of the chlorophyll 

 apparatus in the one and its absence from the other. This 

 involves, however, a change of habit of life which has led 

 to modification of the structure of the plant body. 



It is not very difficult to see how the parasitic habit 

 probably arose in these lowly plants. Their bodies are 

 not differentiated into definite members like the higher 

 plants, but arelitile microscopic spheres or flat plates, 

 or filaments. Many of them now are found to be 

 living in a soit of association with each other, not help- 

 ing each other further than by supplying mechanical sup- 

 port. If we imagine a comparatively large form support- 

 ing a number of smaller ones, we can see that its death 

 and decay would present to those adhering to it a con- 

 siderable amount of organic material ready for consump- 

 tion. Such a source of food supply may well ha\e been 

 utilised, for its absorption would relieve the adhering 

 plants from much labour of construction. A saprophytic 

 habit thus assumed would be likely to be permanent, 

 and the manufacture of the now unnecessary chlorophyll 

 apparatus would gradually die out. 



The forms thus acquiring saprophytism have betn many 

 and varied in their form. The great majority have 

 been filamentous, consisting of long threads known as 

 hyphd . These threads permeate the mass of the decaying 

 organic matter. Such are many of our common moulds, 

 which are developed now so easily upon syruppy sub- 

 stances. 



The passage from this comparatively harmless way of 

 getting food to the destructive form of parasitism is not 

 very difficult We can trace it in many of the fungi 

 which are at our side today. Instead of waiting for the 

 death of the plant to which the fungus is attached, the 

 latter in many cases kills it by secreting and pouring out 

 a toxic substance or poison which causes a local death of 

 the tissue with which it comes into contact. Into this 

 dead nidus the filaments of the intruder then grow, and 

 so its establishment takes place in the interior of its host, 

 such growth being preceded by a destruction of the latter, 

 the materials so formed being the food of the fungus. 



This conduct marks a stage very near to the establish- 

 ment of the true parasitism, which involves only the feed- 

 ing of the intruder on the materials of the host plant, 

 prior to death and decomposition. This change of nutri- 

 tive method soon follows, the intruder gaining the power 

 to assimilate the juices of its host without any such de- 

 composition. Then the gradual weakening of the host is 

 the sign of the in\-adi,'r, which has ceased to manufacture 

 the toxin or poison which was at one period a necessary 

 phase in the process of the nutrition. 



We cannot point to organisms which are at present in 

 the early stages of this transformation of nutritive pro- 

 cesses, but certain fungi can be found which have hardly 

 passed beyond that of the loss of the chlorophyll appa- 

 ratus. One which is known as Pythiitm attacks young 

 lettuce seedlings, causing the disease known as dniiipinf^ 

 of. This illustrates the change ; it has no green colour, 

 and gains all its food from the lix'ing tissue of the seed- 

 lings, but its structure, and especially its modes of repro- 

 duction, are strikingly lik'e those of the alga? to which it 

 is related. The reproducti\e cells which it forms, their 

 shape, and structure, the mode of their formation, andlheir 

 general behaviour are strikingly algal. Among the 

 various species of the genus we find forms whicli are 

 gradually losing these algal peculiarities, andare beginning 

 to show the degradation of structure which always is 

 associated sooner or later withthe parasitic mode of life. 



Besides these pathogenic forms, bacterial and fungal, 

 associated emphatically with a diseased condition of the 

 host plant. Nature shows us others which are much 

 higher in the scale of organisation, belonging iiuleed to 

 the highly organised flowering plants. When we pass 

 in review a series of these parasites we find the same 

 succession of events, the acquirement of parasitism ac- 

 companied by a loss of the power of constructing organic 

 substance and a progressive degradation of the whole 

 organism. 



In tracing the development of the habit among these 

 higher plants we find suggestions that it originated in a 

 different way from that which we have noticed among 

 the fungi. Saprophytism is not unknown among the 

 flowering plants, but it has apparently had no part in the 

 development of parasitism. The origin of the latter 

 must be looked for in the close relationship often found 

 existingamong plants which were originally nothing more 

 than neighbours, by virtue of which they came to help one 

 another in a peculiar manner in the struggle for existence. 

 This relationship, known as symbiosis, is a union of two 

 plants for their mutual benefit. It is seen in many cases, 

 conspicuous among which we have the lichens, peculiar 

 organisms consisting of an alga and a fungus living in 

 close relationship with each other, each contributing a 

 share to the well-being of the compound organism. 



The origin of such symbiosis among the higher plants 

 can be seen in the case of a group of plants often known as 

 roolparasilcs. They include many members of the Natural 

 (_)i(ler ScrophuliU'iiKr, e.g., the yellow-rattle which grow 

 in pastures and waste ground. The roots of these plants 

 are growing freely among the roots of the other plants, 

 the grasses, &c., of the pasture. Coming into contact 

 with these, the irritation of the contact causes a swelling 

 to arise upon the root of the rattle, and from this growth 

 delicate filaments emerge which penetrate the grass root 

 and set up an intimate relationship between the two, 

 which become so far united that liquid matter can pass 

 with comparative ease from the one to the other. The 

 relationship so set up is not particularly harmful to the 

 grass ; indeed, it seems to be beneficial to both symbionts, 

 bringing about in a way an eijualisation of the nutritive 

 material that both are engaged in making. It involves 



