April 1, 1891.] 



KNOW^LEDGE. 



63 



can only effect an entrance to it by way of a tube fin 

 many cases) in the style which opens to the exterior, in the 

 stigma, and communicates below with the cavity of the 

 ovary or o\iile-containing portion of the carpel. The 

 ovule is built up on the same plan as that of the Pino, but 

 instead of being furnished with one integument, it has 

 two, both of which completely enclose it with the excep- 

 tion of an aperture, the micropyle, so that the pollen-tube 

 may be enabled to reach the macrospore. Fig. IV. a. shows 

 a vertical section of an ovule of the Hyacinth (SrUht nutiinx) 

 as it appears before fertilization ; on the outside are seen 

 the integuments, enclosing the wall of the macrosporau- 

 gium. The part by which it is attached to the carpel is 

 called the stalk, or funiculus (./'. ) ; m. is the micropyle. 

 The macrospore is known as the embryo-sac ; towards its 

 micropyle end lie the ovum {<n\) and two cells, synergidoe, 

 (.sy«. ; (Tvv with, and Ipyov work), which are thought to 

 represent an archegonium, as they guide the pollen-tube 

 to the ovum. At the opposite end there are three cells 

 {(int.), termed the antipodal cells. They represent the 

 prothallus which filled the whole embryo-sac in Pinus, 

 and was an independent plate of green tissue in the Fern. 

 It is, then, very much reduced here. This is the tendency in 

 plants : the higher the plant, the greater the reduction of 



Fig. IV. — a. Diflfjframmatic .section of ovule of Srilln nufiins (Wild 

 Hyacinth) previous to fertilization; ///., micropyle; /'., funiculus 

 (stalk); or., ovum; -"yn.. synerj<i(liu ; <in>., antipodal cells; sn , 

 secondary nucleus. 6. Diagram illustratinjj development of embryo- 

 sac (macrospore). c. Germinating Bean {ad. luit.). d. The same at 

 a slightly later stage; r.. cotyledons;.?., young stem bearing ordi- 

 nary loaves (<«/. nat.). e. Diagrammatic section of portion of seed of 

 Grass. 



the prothallus stage of its growth. The central cell is 

 the secondary nucleus. The remainder of the macrospore 

 is filled with nourishing material. 



It will be interesting to note how these cells have origi- 

 nated. We will go back to that stage in which the 

 embryo-sac (macrospore) appeared as a single cell, when 

 it was essentially tlie same as the Fern spore. All cells 

 contain a nucleus, which is probably a part of its proto- 

 plasm that luis become denser in structure, and per- 

 haps changed in some other way. The nucleus is an 

 important body, as in the division of cells it first divides 

 up, and then becomes surrounded with protoplasm. The 

 nucleus of the ciiibryo-sac divides into two. Tlie two thus 

 formed similarly divide, and so do the resulting four 

 nuclei. The positions they assume in each stage are 

 shown in Fig. IV. I>. Then one nucleus from each end of 

 the macrospore travels to the centre. There they fuse 

 to form the secondary nucleus. This stage is represented 

 in the fourth diagram of Fig. I\". //. 



The pollen-grain, when it has alighted on the stigma, 

 commences to germinate, and sends a tube down the 

 style, just as that of Pinus does ; only here germination 

 takes place on the stigma, whereas in Pinus it is effected 

 on the macrosporangium (ovule) itself. The contents of 

 the tube having fu.sed with the egg, the latter di\-ides up 

 to form the embryo. At the same time the secondary 

 nucleus splits up, and so on until the whole of the embryo- 

 sac is filled with a tissue — endosperm, a mass of nourish- 

 ing material. The developing embryo is furnished with a 

 suspensor, as was that of Selaginella and Pinus, and, as 

 in the latter case, development takes place in such a way 

 that the root is directed towards the micropyle, and the 

 first leaf or leaves, as the case may be, in the opposite 

 direction. The reason of this arrangement will soon be 

 obvious. All plants containing green colouring-matter 

 (chlorophyll) obtain a large proportion of their food (at 

 least the whole of their carbon) from the air. But when 

 chlorophyll is absent the process of carbon assimilation. 

 that is, the breaking up of the carbon dioxide of the air 

 and taking up of the carbon and part of the oxygen, can- 

 not go on, so the plant starves. This anyone can test for 

 himself by growing plants in the dark ; then chlorophyll 

 will not be formed, what was previously there will disap- 

 pear, the plant will assume a miserable, starved appear- 

 ance, and will soon die, unless again brought to the light. 

 However, place a potato in a dark, moist cellar (most 

 cellars appear to possess these qualifications), and see what 

 happens. A long, lanky plant will be produced. This 

 will appear to contradict the statement just made ; but a 

 little consideration will show that it only apparently does 

 so, for the potato is a storehouse of nutritive material on 

 which the developing plant has been feeding. \Yhenever 

 this store is exhausted, the plant dies. The ovule partakes 

 in a great measure of the characteristics ascribed to the 

 potato tuber. It is a reservoir of nourishing matter, in 

 which the young plant is bathed at the commencement of 

 its existence ; this material it absorbs before it quits the 

 seed. 



For the examination of a mature seed we will take the 

 common Garden Bean. It shows outside a scar or hiUim 

 that indicates the position of the short stalk which joined 

 it to the carpel, and near the hilum is a small o])ening. 

 the micropyle. Take a penknife and cut the seed-coat, 

 and the contents will then fall out, and probably divide 

 into two thick fleshy lobes, on one of which there is a 

 knob-like process. This small process is the young 

 plant, and the two fleshy lobes (Fig. II. </., c.) are its seed- 

 leaves or cotyledons, as they are ridiculously termed. 

 However, this term must be remembered, as it is used in 

 the naming of two large groups of Angiospermous plants — 

 the Monocotyledons and Dicotyledons. The Monocotyle- 

 dons (/xoi'os one) have only one seed-leaf. Examples of 

 these are found in the Lilies, Hyacinths, Irises, and 

 Crocuses. They are further distinguished by the posses- 

 sion of leaves which have their veins running in a 

 parallel manner. The Dicotyledons (8is twice), on the 

 other hand, have two seed-leaves, and their leaves are 

 net-veined. Our forest trees and many of our common 

 garden plants afibrd us examples of these. The seed-coat 

 is formed by the integuments and wall of the spomngium. 

 The whole of the cavity of the macrospore, or embryo-sac, 

 is occupied by the embryo plant and its enormously large 

 cotyledons. The latter have absorbed the nourishing 

 material that previously existed in the macrospore. A 

 seed which, when mature, contains no nourishing matter 

 is said to be i.rtilliiiiiiiiinu.i. 



We will compare this seed with that of a grass, such 

 as the Wheat ( Fig. IV. <.). -A vertical section shows 



