April 1, 1891.] 



K N O WL EDGE 



61 



J^' AN ILLUSTRATED "^^ 



MAGAZINE OF SCIENCE 



SIMPLY WORDED— EXACTLY DESCRIBED 



LONDON: APRIL 1, 1891. 



CONTENTS. 



A Seed, and What it Contains. By J. Pentland Smith, 



M.A.. B.Sc.&c ' 61 



Numniulites and Mountains. By R. Lydekker G4 



Dissemination of Seeds. By Tiikohoue W. Dicker ... 6.5 



Calcite and Aragonite in Shells By Vaoghan Cornish, 



B.Sc, F.C.S 67 



Excavations at Luxon. By Canon Isaac Taylor 69 



Letters: — A. S. Hansard; T. S. Bakrett ; F.J. Provis ... 70 



Stellar" Spectra. By E. W. Macnueb, F.R.A.S 71 



A Perpetual Calendar 74 



Our Invisible Foes, or Bacteria in Agriculture. By 



Miss A. W. BucKLAND ... ... 75 



The Face of the Sky for April. By Herbert Sadler, 



F.R.A.S 78 



Whist Column. By JIontagd Gattie, B.A.Oxon 79 



A SEED, AND WHAT IT CONTAINS. 



By .J. Pentland Smith, M.A., B.Sc, &c., Lciturer on 

 Botany, etc., Horticultural Collerje, Swanleij. 



A SEED has from time immemorial been taken 

 as the symbol of reproductive power. To 

 explain adequately what are the significations of 

 its dift'ereut parts, we must first occupy ourselves 

 with a preliminary developmental sketch, and 

 will commence with a description of the method of re- 

 production of Axpidiuiii jili.c mas, the Male Shield Fern, 

 one of that large group of plants called Vascular Crypto- 

 gams. Its life-history has previously been discussed in this 

 Magazine. At present we will only touch on those parts 

 of its structure which immediately concern us. 



Brown patches may at times be noticed on the backs 

 of its leaves. These are collections of spore-bearing sacs, 

 or sporangia, that are covered over by a growth of the 

 leaf termed the indusium (Fig. I. a.). The contained 

 spores are all alike. They are minute single-celled 

 bodies filled with protoplasm and nourishing material. 

 The sporangia rupture when ripe, and the spores fall to 

 the ground. There they germinate if the conditions are 

 favourable, and the product of germination is a small 

 green plate of tissue, about one-sixteentli of an inch in 

 length and breadth, and shaped as seen in the diagram 

 (Fig. I. t>.). It is called the prothallus. On its under 

 surface are two kinds of organs, which are of great im- 



portance in the life-history of this fern. These are the 

 archegonia (Fig. I. h., arch., and rf.), or female organs, and 

 the antheridia, or male organs (Fig. I. h., an., and c). 

 The archegonia are flask-shaped. The base of the 

 flask, which is embedded in the prothallus, contains an 

 egg or ovum, the female reproductive cell. The anthe- 



fhrxoL 



Fig. I. — a. Transverse section of leaf of a Fern (As/iidiuui filix 

 7nas), showing sporangia. /'. Under surface of prothallus ; arrh., 

 archegonia ; an., antheridia. c. Longitudinal section of antheridiam 

 (much magnified). (I. Ditto of arohegonium. 



ridia are globular in form. Each one contains numerous 

 lively specks of protoplasm (irpoiros first, and 7rAao-/u,a form, 

 the linng part of the plant-cell, by whose activity the 

 plant is built up), which move about with great agiUty 

 when the antheridia burst. They then make their way 

 (if moisture be present) to the neck of the archegonium ; 

 one of them enters it, and pierces the ovum. Thus 

 impregnation is eftected." 



The result of fertilization is a fern-plant that during 

 its early period of existence sends out an organ called a 

 foot into the tissue of the prothallus, to absorb therefrom 

 the food-material that the plant is as yet tmable to obtain 

 independently for itself. In the meantime it sends roots 

 into the soil and a stem into the air, and when the pro- 

 thallus is exhausted, it is ready to commence life on its 

 own account. 



In Etjukctuw, the Horse-tail, an ally of the fern, the 

 proces.=es are essentially similar, but the male and female 

 organs (the antheridia and archegonia) are borne on 

 separate prothalli, although the spores from which these 

 arise are all alike. 



In a higher branch of Vascular Cryptogams differen- 

 tiation of male and female is carried back a further stage, 

 for two kinds of spores are here produced. Sclugimlhi, so 

 common in our greenhouses, may be selected as a typical 



• Prothalli are often found growing in the crannies of the wall of 

 a greenhouse in which ferns have been standing, but the non-possessors 

 of a greenhouse can easily procure prothalli by sowing spores in a 

 pot of light soil, carefully watering them, and covering the pot with a 

 bell-jar. The length of time the spores take to germinate depends 

 on the species of fern from which they have been derived. Suppose 

 that a prothallus has been found, that the little root-hairs on the 

 under surface have been freed from particles of soil by careful 

 manipulation under water with a camel-hair brush, and that it has 

 been mounted in water on a glass slide, under-side uppermost, and 

 covered with a cover-slip. It is now ready for examination under the 

 high power of a microscope. By gentle pressure of the cover-slip 

 the spermatozoids may he ejected from the antheridia. and m.iy be 

 seen wriggling through the water. 



