120 Practical Plant Biology. 



It is of great interest that this alternation of sexual and asexual 

 generations which we have just studied in Polysiphonia, occurs 

 in less precise and clear-cut phases in many simpler and more 

 primitive plants, while it is found firmly established and with un- 

 failing reiteration in the life-history of all the higher types of 

 vegetation. Nothing strictly comparable to it characterises the 

 life-histories of animals. Its occurrence is one of the broad dis- 

 tinguishing characteristics of the vegetable kingdom. 



PRACTICAL WORK. 



Mount a small piece of Polysiphonia in sea-water. Observe that when 

 the branch is caused to roll over by pushing the cover-glass on one side it 

 is evidently cylindrical. Sketch with low power. With the high power 

 make drawings of it at different levels of focus. The first is to be of an ex- 

 ternal view of a portion of a branch. It will show the outer surfaces of the 

 cortical cells on one side of the branch. Those in the middle line of the 

 branch next the observer will show their complete outer wall, which being 

 viewed perpendicularly will not be foreshortened. The outer faces of those 

 removed from this line, forming the curved surface of the cylindrical stem, 

 are oblique to the line of vision of the observer and appear foreshortened and 

 consequently will have to be represented narrower on the drawing. The 

 number of cells visible on the near side of the branch by focussing at different 

 levels should be carefully noticed. The drawing should also show the 

 manner in which the cortical cells of the adjoining branch-segments inter- 

 lock. The cell contents of the nearest cortical cells should be recorded and 

 careful search made for the protoplasmic communications between the cells. 



The second drawing should represent a median optical section of the 

 branch and should show about three successive axial cells in section and on 

 each side of them radial sections of the cortical cells. Carefully record the 

 dimensions of the cells in both drawings. 



Reconstruct a transverse section of the stem. Being a cylinder its trans- 

 verse section must be a circle. Make a circle having a diameter equal to 

 that of your longitudinal optical section. Within this draw a concentric 

 circle having a diameter equal to that of the axial cell in the longitudinal sec- 

 tion. Now subdivide the circumference of the outer circle into as many 

 divisions as there are cortical cells i.e. double the number recorded in the 

 first drawing. These divisions should equal the width of the middle cortical 

 cells in the first drawing. Draw radii from the points marking these sub- 

 divisions on the outer circle to the inner circle. In this way the outlines of 

 the cells will be obtained as they would be seen in transverse section. 



Search should be made for the reproductive organs, tetrasporangia, 

 antheridia and procarps in their various stages of development. The car- 

 pogonial branches and trychogynes are very difficult to observe. The details 

 of their structure can only be made out by embedding and section-cutting. 



