January, 1913. 



KNOWLEDGE. 



19 



floating aquatic fern, but it has roots, not modified root-like 

 leaves, and its leaves are divided into two lobes, the upper 

 lobe having a small cavity inhabited by the blue-green alga 

 Nostoc. The spore-case groups (sori) are usually in pairs, 

 the microsporangial sori being much larger than the mega- 

 sporangial sori and the latter containing a single megasporan- 

 gium with a single spore. The mega-sorus of Azolla has 

 often been compared with the ovule of a seed-plant, theindusiuin 

 being likened to the integument of the ovule. One of the 

 most striking characters of Azolla is the presence of several 

 masses of frothy substance (massulae) in each microsporau- 

 gium, the massula having the microspores embedded in it 

 while its surface is covered with curious barbed hairs 

 (glochidia). The spongy massulae float about on the surface 

 of the water; the mega-sorus sinks at first, but eventually 

 decay of the indusium sets the spore free and it germinates, 

 producing a female prothallus which rises and floats about on 

 the water, becoming anchored to a floating massula by the 

 barbed glochidia. 



Hannig has investigated especially the structure of the 

 massulae, and of the three curious floating-masses on the top 

 of the megasporangium which are formed in the same way as 

 the massulae. He finds that the substance (periplasmodium), 

 from which the massula material is formed, is derived from 

 the well-developed tapetum layer, the nuclei of which show 

 repeated division. These periplasmodium nuclei lie originally 

 just within the wall of the microsporangium, but later become 

 scattered through the protoplasm ; the periplasmodium 

 increases in volume, stores up starch, and produces a number 

 of vacuoles in which the microspores become enclosed. 

 Within these vacuoles there appears a fine foam-like network, 

 and from the surface of this foam there arise finger-like pro- 

 cesses which become glochidia, developing recurved hooks 

 at their tips. The periplasmodium is originally composed of 

 living protoplasm, but eventually the walls of the foam-like 

 structure become changed into cellulose, while the glochidia 

 walls are cutinised. Hannig also examined the floating- 

 apparatus of the megasporangium, and found that it is 

 developed in exactly the same way as the massulae in the 

 microsporangium ; he detected the thirty-one abortive mega- 

 spores as irregular yellowish inclusions in the meshes of the 

 floating- mass. He confirms the view of former investigators 

 that the unisexual sori of Azolla have arisen from an 

 originally monoecious condition. 



MENDELISM AND PIGMENTATION.— The greater 

 part of the President's address to the Botanical Section of 

 the British Asssociation was devoted to a consideration 

 of the methods of Mendelian research, with special reference 

 to the chemical phenomena concerned in the production of 

 colour in flowers. The Mendelian method is analogous to that 

 of the chemist, who, given a complex mixture, sorts out the 

 ingredients and submits them severally to analysis. The 

 Mendelian analyst, given that complex mixture which is called 

 a plant or animal individual, sorts out the ingredients and 

 analyses them. The Mendelian analysis is made not by direct 

 but by indirect methods ; so long as the physical nature of 

 living substance remains unknown we can hardly hope to 

 resolve an individual into its physical components — all that 

 can be done is to make comparative analyses of individuals, 

 and discover how their several components differ from each 

 other. For this purpose one may represent the individual by 

 the equation, Individual = X + C, where C represents the sum 

 of the Mendelian characters, and X the imaginary or real 

 individual groundwork left after all the Mendelian characters 

 have been removed by analysis. The Mendelian method is 

 concerned directly with the resolution of C into its components. 

 Indirectly, it is also concerned with X ; for if the full value of 

 C be determined, that of X may be inferred. This concession 

 made, it is permissible to concentrate our attention on the 

 term C, and the business of the Mendelian is to resolve this 

 complex of characters into its constituent unit characters. 



As the result of experimental analysis, Mendelism is enabled 

 to state the problem of the behaviour in inheritance of two 

 individuals in the following terms. The complex of character- 



istics which distinguishes an individual is the expression of the 

 sum of a long series of characters. As the individual arises 

 from germ-cells, so each character arises from a germ within 

 the germ-cells — such germs of characters are called factors. 

 When two germ-cells unite to form an incipient individual, or 

 zygote, they bring together the similar factors of a given 

 character — one factor from the one germ-cell and the other 

 from the other. As the zygote forms the individual, so the 

 paired factors give rise to a character of the individual. The 

 body characters may be termed the flowers of the factorial 

 seeds implanted in the germ-cells. 



Some characters are simple and derive from one pair of 

 factors only ; others are of an ascending order of complexity 

 and may be traced to the cooperative agency of two or more 

 pairs of factors. In the case of a complex character the 

 determining factors may be either like or unlike each other. 

 Thus two pairs of different factors are required to produce the 

 character of colour in certain flowers ; but it is probable that 

 certain characters are the outcome of repeated doses of the 

 same factorial stimulant. 



The individual is a dual thing, comparable to a double- 

 barrelled gun, each barrel loaded with the factorial charge 

 supplied by one of the two gametes or germ cells by whose 

 union its duality is constituted. Conversely, a gamete or 

 germ-cell is of single and not of dual nature ; it has only one 

 barrel and therefore can give effect to only one of the two 

 factorial charges with which the individual was supplied at the 

 time of its formation. The image of the double-barrelled gun 

 serves to illustrate the several states in which an individual 

 may exist with respect to its charge of factors of any given 

 simple body character. Both barrels may be loaded — an 

 individual in like state has two factorial charges and produces 

 gametes all of which are alike in the possession of one of these 

 factors ; such an individual when self-fertilised, or mated with 

 its like, produces gametes which are all alike in this respect, 

 and these gametes, fusing in pairs, give rise to individuals 

 which all possess the character in question; such individuals 

 are homozygous, they breed true to the character. Neither 

 barrel may be loaded — an individual in like state is also 

 homozygous, it breeds true to the absence of the character. 

 If a gamete of the former individual meet with one from the 

 latter individual, the resulting zygote is like a double-barrelled 

 gun with only one chamber loaded ; the zygote is heterozygous 

 for the character, and unlike the homozygotes, which breed 

 true, the heterozygous individual does not breed true to the 

 character in question. 



From these propositions it may be predicted that the 

 offspring of the heterozygote fall into three groups — one 

 homozygous for the character, another heterozygous, and a 

 third homozygous for the absence of the factor ; and that 

 these types of individual occur in the proportion of 1:2:1. 

 This prediction is verified by experimental breeding from the 

 heterozygote. 



The Chinese Primrose, Primula sinensis, has given rise to 

 many distinct varieties, white and coloured. It would appear 

 self-evident that the white races differ from the coloured races 

 merely in lack of pigment, but Mendelian analysis shows that 

 there are more subtle differences between the different races. 

 These differences become apparent when true-breeding white 

 and coloured plants are crossed with one another ; for it is 

 then discovered that two types of white-coloured plants exist, 

 and it is only by their fruits — their offspring — that we may 

 know them. Thus, if certain white-coloured races are chosen 

 for the experiment, the result of crossing white and colour is 

 that all the offspring of the cross bear coloured flowers. If 

 certain other white races are used and mated with the coloured 

 form, however, the offspring all bear white flowers. In the 

 former case, where the first generation (Fi) consists of coloured 

 offspring, the second generation (F 2 ) raised by self -fertilising 

 F individuals by crossing with one another, consists of coloured 

 and white in the proportion of three to one ; hence we conclude 

 that the white used in this case owes its character of white- 

 ness to lack of the pigment-producing factor which is present 



