44 



KNOWLEDGE 



[Janhaky 1, 1890. 



appear the archegonia, which are the female 

 elements of reproduction. Farther back, nearer to the 

 point, so to speak, of the heart, 

 still on the under surface, we find 

 the root hairs, and among them 

 the male elements, the anthe- 

 ridia. The development of these 

 latter, as it is simpler, may be 

 described first. They take their 

 Fig. 3.-SECT10S of Pro- qj.j^ f^.^j^ ^^^^ ^f tj^g gglls 



THALLIUM AND Young bERN. J. ■ ,, 14 f J.1 „ „ 



A. Proth.-,nmm; B, First formmg the plate of the pro- 

 frond ; c, First root. thallium, as a sunple prolon- 

 gation downwards, just as hairs 

 grow on the stems or other parts of plants. Soon, 

 however, in the middle of the free extremity of each of 

 these, is seen a peculiar round body, of a different colour 

 to the rest of the cell. These bodies rapidly surround 

 themselves with a distinct cell-wall, and subdi\-ide into a 

 number of small cells. In these latter the antherozoids 

 are formed, each consisting of a spirally -coiled thread, 

 thickened at one end, and likewise at that end furnished 

 with six long cilia. In due course the antheridia burst, 

 the ceDs containing the antherozoids also open, and their 

 contents, which move actively, are discharged. A few 

 brief remarks on the comparative position and relation of 

 these antherozoids will perhaps be in place here before 

 proceeding to describe the development of the archegonia. 

 In one of the lowest groups of Algse — the Zygospore* — 

 sc.rwil reproduction as distinguished from vegetative repro- 

 duction is effected by the coalescence of two cells to form 

 one, which is called a zygospore to denote its origin. No 

 difference whatever can be observed between the two cells 

 which thus come together ; we cannot definitely say that 

 one is male and the other female. At the same time, the 

 mobility of the antherozoid is evidently foreshadowed in 

 the cell which travels towards and pours its contents into 

 the other ; and this latter may be considered as represent- 

 ing, even in a rudimentary degree, the female egg-cell — the 

 oosphere. This forms the earliest commencement of a 

 differentiation, or unlikeness, between the male and female 

 elements ; a differentiation which proceeds more or less 

 regularly upwards to the ferns, perhaps its highest point, 

 whence it declines to the flowering plants. 



Passing now, with a wide leap, to flowering-plants, we 

 shall see why the word "declines" was used above. In 

 them the pollen-grain, alightmg on the stigma, puts forth 

 the pollen-tube into the cellular mass of the stigma. The 

 tube lengthens, and passing through the loosely-compacted 

 tissue of the style, assisted, as has been suggested, by cer- 

 tain projections on the walls of the ovary, it eventually 

 penetrates the micropyle of the ovule. Fertilisation is 

 effected by a certain amovmt of the contents of the tube 

 fusing with those of the two cells which lie nearest the 

 entrance — the micropyle — and so passing into the oosphere 

 beyond. Before this takes place, however, the protoplasm 

 of the pollen-tube sej)arates itself into an indefinite number 

 of nuclei. These nuclei are the traces, as it were, of the 

 complete cell-formation, which has been described in the 

 development of the antheridia ; they are the portions of 

 the male organs of reproduction in flowering-plants which 

 represent, in a degraded state, the antherozoids of which I 

 have been speaking. Thus, fi-om the undifferentiated cell 

 of the humble alga to the highly organised antherozoid of 

 the vascular Cryptogams, we can trace out the successive 

 steps in that differentiation ; while we can make out, with 

 more diiiiculty but yet with comparative certainty, the 

 stages wliich lead downwards from the antherozoid through 

 the gymnosperms to the mere division of protoplasm in the 

 pollen-tube of angiosperms (flowering-plants). The arche- 



gonia of the prothallium of a fern are nearly allied in form 

 to those of the neighbouring order, the mosses. Each con- 

 sists of a ventral portion, imbedded in the surrounding 

 tissue and coherent with it, and of a short neck, througli 

 which two canal cells, as they are called, which sul)sequently 

 undergo disintegration into mucilage, extend to the 

 egg-cell. 



Before describing the development of these archegonia, 

 a brief mention of the comparative anatomy of these canal- 

 cells may be of interest. In animal embryology, you will 

 remember, during the gi'owth of the egg-cell, or ovum, 

 that is to say, during the series of changes whereby it 

 becomes ripe for impregnation by a spermatozoon, two 

 peculiar bodies, called polar bodies, are thrown off'. These 



Fig. 4.-AECHEGOXIUM. „, piG. 5. - Antheridium. a, 



lere; p. Canal-cells; y, Antherozoids; B, Chlorophyll 



Chlorophyll corpuscles. corpuscles. 



polar bodies are derived ft-om the original nucleus of the 

 egg called the germinal vesicle, and the portion of it left 

 behind in the 0M.im is termed the foinn If pronurleus. In the 

 archegonium of a fern the canal-cells, so early severed 

 from the egg-cell, correspond to these same polar bodies in 

 animals, and the only explanation of the why and where- 

 fore of this proceeding in the animal and vegetable 

 economy is that given by the late F. M. Balfour. It is as 

 follows : — After the formation of the polar cells (and the 

 separation of the canal-cells), the remainder of the ger- 

 minal vesicle within the o\iim is incapable of further 

 development without the addition of the nuclear part of 

 the male element (spermatozoon or antherozoid), and he 

 accounts for the prevalence of such a contrivance among 

 plants and animals by assuming that it is to prevent the 

 ovum developing by itself without fertilisation, somewhat 

 analogous to the arrangements m plants to prevent self- 

 fertilisation ; the latter being no worse, to say the least, for 

 the plant than the former. 



The most frequent place for the spermatozoon to 

 penetrate into the ovtim is just where the polar bodies are 

 formed ; it seems as though they were guided by their 

 presence, or the trace of them on the outside. .lust 

 so through the canal-ceUs the antherozoid has to make 

 its way to the oosphere within the archegonium, which 

 is composed of a layer of transparent cells, arranged in 

 four tiers, surrounding a central ca^^ty which extends 

 into the thickened portion of the prothallium. Each 

 one is developed from one cell of the prothalUum, which 

 swells up above the surface, and is soon separated into 

 two by a horizontal partition. The upper of these two 

 cells thus produced gives rise, by repeated di\nsions, to 

 the neck of the archegonium, which, when fully developed, 

 is composed of about twelve cells, built up in rings of four, 

 one upon another, so as to form a kind of chimney-shaft 

 having a central passage leading down to a cavity at its 

 base. The lower of those two first formed cells becomes 

 the central cell of the archegonium and, again dinding, 

 cuts off' two or more cells wliich fiU up the neck, the canal 

 cells so often mentioned above ; while the largest and 



