228 THE USE OF THE MICROSCOPE 



24. Formation of Spenn Nuclei in Pollen Grains or 

 Pollen Tubes. — Cover the stigma of Datura, etc., with 

 pollen. When the grains have just germinated, press 

 them slightly under a cover-glass in iron-acetocarmine in 

 which just enough crystals of chloral hydrate have been 

 dissolved to make the pollen grains become transparent. 

 Seal with melted parafhn. Examine after a few days, 



25. Extra Chromosomes of 2n + 1 Plants. — Such 

 trisomic plants may be detected in the progenies of Datura, 

 Nicotiana, Matthiola, Lycopersicum, etc., grown from 

 buds exposed to changes of temperature. Or they may 

 form most of the progeny of triploids pollinated by diploids. 

 Prepare as in 3. The chromosomes and cytoplasm may be 

 squeezed from the cells. Proofs, as in 20, that the extra 

 chromosome of the trivalent, or the univalent, does not 

 divide at the first division, may be looked for. (Belling and 

 Blakeslee, Clausen, Frost, Lesley.) 



26. Deficient Chromosomes in 2n — 1 Branches of 

 Datura, Etc. — These branches can be recognized by their 

 different appearance, and (in Datura) by the flowers having 

 half or more of their pollen grains empty. Preparation of 

 pollen mother cells as in 3. Proofs are to be found (in 

 Datura) that the univalent chromosome does not divide 

 at the first division, but does divide at the second division, 

 (Belling and Blakeslee, Clausen and Goodspeed.) 



27. Assortment of Chromosomes in Haploid Datura, 

 Nicotiana, Triticum, or Crepis.— Haploids occur in the 

 progeny of plants whose buds have been chilled and then 

 warmed; or they occur in attempted distant crosses. 

 Preparation of pollen mother cells as in 3. The assortment 

 is to be studied at second metaphase. Six sizes can be 

 distinguished in Datura. The total of both second meta- 

 phase groups in Datura is twelve. Hence no univalents 

 divide here at first metaphase. The other haploids may 

 be tested for this. (Belling and Blakeslee, Clausen and 

 Goodspeed, Gaines and Aase, HoUingshead.) 



28. Fracture of a Chromosome in Secale, — Plants with 

 eight pairs of chromosomes seem to occur normally in fields 



