INTRODUCTION TO THE METHOD 



All these methods and others will meet us in the chapters which follow, but in 

 order to give the reader some preliminary insight into the type of observations actually 

 involved, it may be helpful to reproduce some of the photographs used in the analyses 

 of Biscutella and Nasturtium listed above. 



Biscutella laevigata L. is a fragrant little yellow-flowered member of the cabbage 

 family, well known to tourists in Switzerland, France, Germany and Spain on account 

 of its curiously shaped fruits (Fig. i), which the Latin name oi Biscutella compares to 

 two shields, although at least one popular name ('Brillenschoten' in German) makes 

 the comparison rather with a pair of spectacles. The species B. laevigata does not 

 occur in Britain, but a number of different strains of it are met with as limestone rock 

 plants in the lowlands or in subalpine meadows in the mountains of central and 

 southern Europe. It is, however, by no means uniform throughout its range. The 

 German and French lowland types have i8 chromosomes except in their pollen grains 

 and embryo sacs, where the reduced number of 9 is found. In Switzerland and Austria, 

 on the other hand, the plants all have twice as many chromosomes, 36 being found 

 in their roots and 18 in most of their reproductive cells. Both types are, however, still 

 interfertile, and hybrids between them, possessing the intermediate chromosome 

 number of 27, are spontaneously formed when suitable plants are grown together in a 

 garden. Fig. 2a-c shows the somatic chromosome numbers of these three types of 

 plants, which may be taken as the first illustrative example of a polyploid series. The 

 number 9 which they all share in varying degrees is the gametic number of the lowest 

 member. This gametic number, which is fundamental to the whole series, is con- 

 veniently designated the monoploid number, in respect of which the plant with 18 

 chromosomes (Fig. 2a) is diploid, the one with 27 (Fig. 2b) triploid and that with 36 

 (Fig. 2c) tetraploid. 



Chromosome pairing at meiosis in diploid, triploid and tetraploid B. laevigata is 

 shown in Fig. 2d-f. In the diploid (Fig. 2d), pairing occurs in the simplest manner 

 possible and nine pairs can easily be seen in the photograph. In the tetraploid of 

 Fig. 2/, however, pairing is more complex, for the presence of four instead of two 

 monoploid sets of chromosomes has led to the formation of numerous quadrivalent 

 groups easily recognizable as such, where the four component chromosomes of a 

 quadrivalent are joined in a ring, as may be seen in many places to the right of the 

 figure. In the triploid (Fig. 2e), where there are three duplicate sets of chromosomes, 

 trivalents and not quadrivalents are formed. These are also easily recognizable by 

 their shapes, and in the cell figured there are five trivalents, four pairs and four uni- 

 valents, the pairs and univalents representing potential trivalents which have fallen 

 apart at an earlier stage into 2 + i . Falling apart into lower valency components is 

 liable always to affect a certain proportion of potential multivalent groups, since the 

 successful cohesion among a group depends not only on homology (that is ability to pair) 

 but also on the number and relative positions of the chiasmata which form after 

 pairing has taken place and by means of which cohesion up till metaphase is made 

 possible. Since the position of chiasmata is, to some extent, determined at random, the 

 precise numbers of effective multivalents which appear at metaphase will vary some- 

 what from cell to cell. The numbers of multivalent groups visible in Fig. 2e and /are, 



