1 1 2 Practical Plant Biology. 



the offspring developing from the spore. On the other hand, there 

 is double the chance of variation in the individual which comes 

 from the oosperm. These two results tend in different directions 

 and on the whole we may look for greater numbers of variations 

 in sexual reproduction, while we may expect among the individuals 

 produced from spores wider and more unbalanced differences. 



Thus we see that sexual reproduction, possesses certain qualities 

 which are not shown by asexual reproduction and under certain 

 conditions these qualities may well be advantageous, and so have 

 become fixed in certain organisms. 



In order to introduce new qualities by sexual reproduction it 

 is evidently advantageous that the sexual cell should originate from 

 plants as widely separated as possible. In stationary organisms 

 like plants this demands that the gametes themselves should be 

 motile. As the organs of motility are cilia the cells must be small 

 and hence the supply of reserve material in the motile cell is 

 limited. A compromise securing enough stored food material for 

 the developing embryo and an introduction into its constitution 

 of the foreign element is secured by the differentiation of the 

 gametes into the large quiescent ovum charged with store 

 materials and the active travelling sperm coming possibly from 

 a distance, or from an individual or organ possessing a different 

 genealogy from the ovum. 



PRACTICAL WORK. 



Cut transverse sections of the receptacles of Fucus platycarpus. Place 

 these sections in a staining block containing salt water. Select out the 

 thinnest and mount in a drop of sea- water and cover. Notice how the 

 cell-walls of the medullary cells swell up and become gelatinous. Sketch 

 the elements of the various tissues, cortex and medulla. Observe and 

 draw the conceptacles. Good complete median sections of the conceptacles 

 are hard to obtain and usually must be built up from many fragments. The 

 walls, the hairs, the antheridia and the oogonia are easily identified. The 

 antheridia are on shrub-like hairs formed of branching filaments of cylindrical 

 cells which support the sac-like ovoid antheridia with coarsely granular 

 contents, which are yellow when mature. The oogonia are the most 

 prominent objects in the preparation. The mature oogonia show the 

 eight rounded ova enclosed in the transparent membranes. The ova are 

 olive-brown. Stages between the first beginnings of the oogonium up to 

 the formation of the ova are easily found and should be drawn. 



Ripe sperms and ova are often found in these preparations or they may 

 be collected by immersing mature receptacles in a small quantity of sea- 

 water. In this manner the act of fertilisation and the first stages in the 

 development of the embryo may be studied. 



The dimensions of the ova and sperms should be estimated by means 

 of the Ghost-micrometer, 



