108 



KNOWLEDGE 



[June 1, 1891. 



elaborated experiments he was forced to confess that no 

 such thing as spontaneous generation takes place — at least, 

 that it cannot be proved to do so. 



" It is easy to understand how such ideas of spontaneous 

 generation should have been prevalent in ancient times. 

 Even their repeated occurrence in modern times, and with 

 our modern knowledge, is also capable of explanation. It 

 must be assumed that organisms did once come into being 

 of themselves without parents, being produced from 

 organizable, but yet not organized, matter. It must, more- 

 over, be allowed that this may still happen at any moment, 

 and perhaps does actually happen ; its impossibility 

 cannot be proved. To produce actual proof of an original 

 formatio)! of a living being is a matter of the highest 

 interest, and has as powerful an attraction for the biolo- 

 gical investigator as the prospect of producing the homun- 

 culus in the phial for the alchemist. But the experience 

 of centuries has shown that wherever the homunculus 

 really appeared in the flask it pi-oved itself to be a small 

 imp which had been secretly introduced into it from 

 without ; and, speaking seriously, the result was always of 

 this kind. In every single instance exact investigation 

 has shown that the organisms which were supposed to 

 have had no parents proceeded from germs produced from 

 parents of the same species as themselves ; it has also 

 been shown how they were formed and whence they came. 

 . . . That there is no generation without parents is a 

 matter of experience ; it is in distinct accord with the 

 present state of our knowledge after making allowance for 

 all conceivable possibilities . . ." 



What, then, are the organisms which have given rise to 

 such curious notions regarding their mode of origin ? We 

 have instanced the nuishroom as an example ; and to it 

 the toadstools and moulds may be iilded. 



The vegetable kingdom is generally divided into five main 

 divisions, of which the lowest is the Thallophytes (6'u\Xos 

 " a young shoot," and <^i'toi' " :• plant "). The plants of 

 this class .show no diilerentiatiou mto root, stem, and leaf, 

 as is the case in the majority of green plants, but instead 

 are composed of a tliallus, or body in which no distinction 

 can be made between stem and leaf, and in which a root, 

 such as we are accustomed to in the higher plants, is 

 absent. In this division the Fungi find their place. 



It would be impossible in the limits of a single paper to 

 attempt a general description of even the chief members 

 of the various divisions of this enormous group. It will 

 best serve our purpose to confine ourselves to a detailed 

 account of the life of an individual member of one of 

 these. The physiology of the Fungi will, we trust, form 

 the subject of a sub.sequent paper. Here it will merely 

 receive a passing glance. 



We select for our description a Fungus which causes 

 ruttt, or mildew, on grass. Its life-history is extremely 

 complicated, and is intensely interesting. It is a typical 

 luxus natuni' of the old authors. The difierent stages in 

 its interesting life may be traced (now that they are known 

 to us) with comparative ease owing to the large size of its 

 members, so that the readers of this article can thus 

 obtain a first-hand knowledge of some at least of its life 

 processes. 



This fungus is the possessor of three names — I'lnln 

 linc/inn, I'uicinin //nmiivis, and .V.ridhim hcyheridix \ the 

 reason of which will be evident to the reader by-and-by. 

 Puccinid i/i-fimhii.i is the one in common use. 



The most conspicuous portion of the fungus, and this 

 holds good for almost all these plants, is that portion set 

 apart for the reproduction of the species. The mushroom 

 is a case in point. The vegetative portion — that part, in 

 other words, which is the carrier-in and elaborator of 



nourishment — is quite inconspicuous. We will deal with 

 it first. It is the fungus body, the thallus or mycelium 

 {/j.vioj'i " a mushroom "), and corresponds to the " spawn " of 

 the mushroom. It consists of a number of elongated, 

 many-celled threads or hypha^ {vcferi "a web"), which branch 

 and interlock to form a more or less compacted weft of 

 tissue. I'lici-iiiiii iiraminis is a pai'asite, so we must look 

 for this mycelium in the tissues of a living being, or host, 

 as it is termed in technical language. 



There are five or more unwilUug hosts favoured with this 

 unwelcome guest. We will select one of these — 'I'litivum 

 vuliiiiir, the Wheat — for our study of the parasite. It 

 occasionally happens during the early summer that a 

 farmer espies a peculiar colouration on the blades in 

 some parts of his wheat-field. He calls this iiiilihn-. 

 Closer examination reveals the fact tliat the blades have a 

 rusty appearance, the rust taking the form of small linear 

 patches on the leaves. A microscopic examination of a 

 transverse section of a leaf through one of these patches 

 shows that its tissue is permeated by a number of delicate 

 threads (Fig. I. J., in.), which run amongst the cells. At 



Vm. I. — o. Diaj^ramniatic representation of surface of leaf of 

 Wheat {Tn'tirum !'i(/'/«ri) .slightly magnified. The uredo-gonidia 

 (iir. (/.) are seen shining ihrongh. //. Transverse seclion of leaf of 

 Wheat through a uredo-gonidia patch. Ur. //., uredo-gonidia ; ep., 

 epidermis; m., mycelium, the threads (/i^/<//«') of which are woven 

 into a loose tissue. (Much magiiilied.J r. Uredo-gonidiuui. (Very 

 much magnified.) 



one portion the filaments have assumed an upward direc 

 tion, and there a mass of yellowish cells have made their 

 appearance (Fig. I. /'., '"'. //.). They are so numerous 

 that the epidermis or skin of the leaf has been ruptured. 

 Each one occupies the end of one of the threads. These 

 threads are the hyphiv of the fungus mycelium. They 

 form the thallus, which corresponds to the root, stem, and 

 leaves of a higher j)lant, such as its host-plant, the Wheat. 

 Many of the wheat-cells are stored with green colouring- 

 matter, or chlorophyll, which enables it to lead an inde- 

 pendent existence," but in the thallus of the Puccinia 

 there is no gi-een colouring-matter ; hence it must live on 

 matter which has already been elaborated from inorganic 

 materials by its more favoured host. So far as regards 

 structure, a ditt'erence exists between the two, occasioned, 

 of course, by the very difierent mode of life of the two 

 forms. In the Wheat-plant certain cells are set apart for 

 the performance of certain functions. Each xet of cells is 

 called a tissue. No such distinction can be drawn between 

 the cells of the fungus mycelium ; they are all alike. 

 Each one can take in nourishment for itself from the ceils 

 of the leaf in which it has found a home. 



Of the yellow cells we have not spoken. They are 

 called gonidia or conidia-gonidia (yciTu'w " to produce," and 

 oiSa " like "), because they are like spores ; true reproductive 

 cells ; and conidia (kovis " dust "), from then- dust-hke 

 appearance. They are not spores, inasmuch as they have 



* For explanation, see article on '• Breathing Organs of I'lants,'' 

 Knowledge, Nov. I.-^IIO. 



