62 



KNOWLEDGE 



[April 1, 1891. 



example. The male spores are much smaller than the 

 female ; hence thi'ir respective names, microspores and 

 macrospores (/ttxpos small, /xuxpos large). The sporangia 

 which contain these are termed microsporangia and 

 macrosporangia respectively (Fig. II. n.). When the 

 macrospore (iim. sp.) germinates, the protluillus does not 

 come out of the spore, but remains as a mass of tissue, 

 witliout green colouring-matter inside it, and in addition 

 there is a nourishing material called endosperm, which 

 later on becomes divided up to form a tissue of cells (Fig. 

 II. /).). The male prothallus also remains inside the 

 microspore. It consists of only one cell, and has no 

 chlorophyll. A peculiar organ, termed the suspensor, 

 develops from the fertilised egg, as well as the true 

 embryo plant. It pushes the embryo into the endosperm, 



avckeojpnxo. 



Fig. II. — a. Longitudinal section of apex of sporangium bearing 

 shooi oi Sclac/ijiel/a; sp., developing sporangia ; iiii. .'c/j. , microspores ; 

 7iia.sj}., macrospores. 0. Diagrammatic section of germinating ma- 

 crospore. c. Ditto of archegonium (much magnified.) 



so that it may feed on it. It is noteworthy that the 

 gi-owth of the embryo takes place iiimle the spore, not 

 outside, as in the two former cases. 



The cone of I'iniis (e.g. Pinns si/lvestris, the Scots Fir), 

 is a collection of leaves that have become modified to bear 

 ovules, which afterwards are termed seeds. The ovule is 

 a macrosporangium which, however, differs from that of 

 SelagineUa in that it possesses a covering or integument 

 (Fig. III. /(., //(.). The whole of the interior of the ovule is 

 occupied by the prothallus. The male spore is familiarly 

 spoken of as the pollen-grain, and the microsporangia as 

 the anther-lobes or pollen-sacs ; they are borne on the 

 under surface of the stamens (Fig. III. a., st., p.) The 

 pollen-grain is provided with two bladders or floats, and 

 is carried by the wind to the ovules which lie exposed on 

 the upper surface of the carpels or modified leaves on 

 which they arise. The integument does not quite enclose 

 the ovule, but leaves a small aperture, the niicropyle 

 (fjiiKfxii small, and ttuXt; a gate) for the reception of the 

 pollen-grain. There the microspore (pollen-grain) enters 

 and rests on the surface of the macrosporangium, germi- 

 nates, and sends a tube (Fig. III. c. d.,f.; and /-., p.) into 

 the microspore, where it comes into contact with the ovum, 

 in one of the archegonia developed in the prothallus. 



There is no endosperm in Pinus, as in SelagineUa, but 

 the important points to be noted are that not only does 

 the embryo develop inside the macrospore, but the macro- 

 spore remains inside the macrosporangium that is still 

 attached to the parent plant, from which it derives suste- 

 nance ; for after the developing embryo has used up all the 



nourishment in the prothallus, fresh nutriment is poured 

 into the macrospore from that source. 



When the tube of the pollen-grain, which we may com- 

 pare to an autheridium, touches an ovum in one of the 

 archegonia, it sends part, at least, of its contents into it. 

 As a result the ovum divides up to form a young plant 

 (Fig. III. c), that is provided with a long suspensor. 

 The embryo is so placed that its root lies towards the 

 micropyle. The mature ovule (macrosporangium) with 

 its coat is the ■■urd, and the collection of seed-bearing 

 leaves which we find form what is commonly called a 

 flower. 



The Angiosperms iayytiov a vessel, and a-Ktpp.a a seed) 

 are so called from the fact that their seeds are enclosed 

 by the leaves (carpels) on which they are borne ; and in 

 this they ditier markedly from the Gymno- 

 sperms lyu/ni'os naked), such as the Pine, in 

 which these are quite exposed. The majority 

 of our familiar garden shrubs and forest trees 

 belong to the large group of Angiosperms. 

 A carpel may bear one or many ovules, which 

 later on develop into seeds, and the carpel . 

 itself undergoes, as a rule, a characteristic 

 change to form what we know as the fruit. 

 The pollen-grains are found in sacs (anther- 

 lobes or pollen-sacs)" situated on the upper 

 portions of the stamens. 



They are very similar to those of the Pine ; 

 but instead of having balloons they often possess 

 warts or prominences by means of which they 

 may adhere to the various animals that carry 

 them to the stigmas of the carpels on which 

 they must alight in order that they may be in 

 a position to accomplish their role in life. 



^^'e have reserved until this stage a minute 

 examination of the macrosporangium, or ovule — the organ 

 that is of such vital importance in connection with the repro- 

 duction of the plant ; the reason being that it is best to take 

 for this purpose familiar examples such as may be easily 

 procured by anyone desirous of making the examination of 



Fig. III. — a. Diagrammatic longitudinal section of male cone of 

 Pinus (Scots Fir) Only two of the pollen-sacs are represented con- 

 taining pollen-grains (microspores), p. ; St., stamen, with pollen-sac 

 on under surface, h. Ditto of ovule of Pinus ; ;;., pollen-grain germi- 

 nation ; Hi., micropyle; in., integument (coat) of macro-sporangium 

 (oTule); sp., WctII of macrosporangium. r. Pollen-grain; _/'., cell 

 ■which will develop into pollen-tube d. The same, wit^h cell (/") de- 

 veloped as pollen-tube. c. Diagrammatic representation of develop- 

 ing egg ; there ought, in reality, tc be four embrj^os figured arising 

 from these ovules. 



the object for himself. These are best found in the butter- 

 cups and daisies, the hyacinths and lilies of our fields, 

 woods, and waysides. In all these cases the ovule is con- 

 tamed within the carpel or carpels, and the pollen -grain 



* See article on " Some Curious Modifications to Prevent Self- 

 fertilization." — Kxowi.KDGE, Feb. 18fll. 



