40 Practical Plant Biology. 



In very simple forms like Chlamydomonas there appears to be 

 no hard and fast line between the spores and the gametes. Thus 

 there are often as many as eight spores formed in the mother-cell, 

 and&s few as eight or sixteen gametes. It has been found that 

 while spores are formed when the supply of nutriment is plentiful, 

 starvation conditions tend to produce gametes. Again, it has been 

 found possible to obtain new individuals from single gametes by 

 furnishing them with rich supplies of food. 



These observations have led to the speculation that sexual 

 reproduction originated in primitive organisms as the result of 

 starvation, and that the fusion or fertilisation which is character- 

 istic of sexual reproduction is comparable to the ingestion of one 

 cell by another in response to the stimulus of hunger. 



PRACTICAL WORK. 



Put a drop of water containing Chlamydomonas on a clean slide, cover with 

 a clean cover-glass, and examine with the low power. Notice the unsteady 

 motion of some of the cells ; in most cases it appears to tend towards the 

 observer. In reality the motion is away from the observer and towards the 

 light. The motion appears reversed owing to the reversal of the microscopic 

 image. These travelling cells rotate on their axis as they advance. Other 

 cells are seen which are stationary: and again others which show an erratic 

 vibratory motion. Put the Ghost-micrometer in position and estimate the 

 size of the cells in hundredths of a millimetre. 



Now take a very small drop of the water containing Chlamydomonas and 

 add it to a drop of i per cent, gelatine on a slide and cover. The colloid 

 gelatine restrains the activity of the cells and makes them move more slowly 

 so that it is easier to observe their motions. 



Select one of the stationary but vibrating cells. Arrange it centrally in 

 the field with the low power. Put on the high power and examine. Note 

 the cell-wall, the coloured region, the colourless beak, the nucleus, the 

 pyrenoid, the stigma, the cilia, and the contractile vacuoles. By careful 

 focussing observe the rounded near surface of the cell. By rapid minute 

 changes of the fine adjustment, try and make out the form of the seemingly 

 coloured protoplasm. By the manipulation of the condenser, mirror and fine 

 adjustment, find and examine the cilia. They are only visible by their differ- 

 ence in refringencyfrom the surrounding water. When seen at their clearest, 

 they appear like very fine filaments of glass. Focus for the median optical 

 section and carefully draw the cell, putting in all the details observed. 



Put on the low power and apply a drop of liquor iodi to the side of the 

 cover-glass, taking care not to allow it to flood the upper surface. When the 

 reagent has penetrated well under the cover- glass note the darkened appear- 

 ance of the cells it reaches. Centre one of these and switch on the high 

 power. Observe the olive-green colour of the chloroplast. The protoplasm 

 has become light brown. The cilia have become much more clear. The 

 granules of the nucleus are considerably darker than the surrounding proto- 

 plasm. The pyrenoid has become dark blue. The blue coloration, in many 

 cases, appears as a shell enclosing a dark brown material. The blue colora- 

 tion is the characteristic reaction of starch with iodine, and the central brown 

 mass is protein. 



