2O2 Practical Plant Biology. 



These are the pollen-mother-cells. The nucleus of each divides 

 twice and produces four nuclei. Round each nucleus the cytoplasm 

 segregates and forms a spherical cell. A cell-wall is formed cover- 

 ing each, and from the remains of the cytoplasm of the mother-cell 

 the cutinised layer and bladders are formed. The four pollen- 

 grains arise as a tetrad in a mother-cell, in a manner quite similar 

 to the spores of the liverworts like Marchantia, the mosses like 

 Funaria and the ferns like Aspidium and Selaginella. Thus, con- 

 sidered from the point of view of development, the presumption 

 that the pollen-grains are spores gains probability. At first the 

 pollen-grain has a single nucleus. It divides and one half becomes 

 enclosed in a very small flat cell which adheres to the inside of the 

 wall of the pollen-grain. The other occupies a central position in 

 the pollen-grain. The small adherent cell collapses and is reduced 

 to what looks like a minute cleft in the wall of the grain. The 

 nucleus of the pollen-grain again divides and the same fate over- 

 takes the second cell formed as the first, and all that persists of 

 these two cells are two clefts or cracks in the substance of the wall 

 of the pollen -grain. The third cell, formed in the same manner 

 however, persists and shortly afterwards it divides to form the stalk- 

 and the body-cell mentioned above. At the time of the dehiscence 

 of the pollen-sac these two cells have just been developed. 



PRACTICAL WORK. 



Saw from a branch of Pinus silvestris ten or more years old a piece of 

 about i centimetre long. Split it into five or six sectors and immerse them 

 in spirit. After ten or twelve hours or longer, change the spirit for a mixture 

 of spirit and glycerine, about half and half, and leave for twenty-four hours. 

 This renders the wood more easy to cut. Split some of the sectors tangenti- 

 ally in the wood and some tangentially in the bast. Smooth all the cut 

 surfaces with a sharp razor and prepare thin sections from the transverse, 

 radial, and tangential surfaces. Place the sections in water and select some 

 of the thinnest of them for staining. Lift these latter into a mixture of 

 watery saffranine and Delafield's haematoxylin and stain for fifteen to thirty 

 minutes. Wash in several changes of spirit and finally clear in oil of cloves. 

 See that the sections remain completely submerged in the oil. When the 

 sections appear quite clear this may be tested by observing them in a strong 

 light before a black background lift them from the oil and lightly touch them 

 against blotting-paper (to remove the free oil), and place them in a drop of 

 Canada balsam on a slide, and cover. During the staining of these sections 

 mount and examine others in water. 



Carefully examine the three kinds of sections, and sketch, using the high 

 power, portions of the different tissues, viz. of the wood, bast, cortex and 

 medullary rays, as seen in all three. 



By careful comparison of the appearances presented in the three sections 

 reconstruct a medullary-ray-cell and draw it in perspective, marking the 

 length of each of its sides as determined by the Ghost-micrometer. 



