INTRODUCTION TO THE METHOD 



fruit. The genus Nasturtium thus now contains not one single species as had previously 

 been thought but two separate species, one of which is older than the other, and partly, 

 though not wholly, parental to it. The two species from the taxonomist's point of view 



Fig. 3. Polyploidy in watercress [Nasturtium), a after Manton (1935), the rest after Howard and 

 Manton (1946). a. A diploid root in the true watercress [N. officinale R.Br.) showing 32 chromo- 

 somes, from a section stained in gentian violet, x 2000. b. Tetraploid cell in prophase from the 

 tapetum of a plant derived from the diploid by colchicine treatment showing 64 chromosomes, 

 permanent acetocarmine. x 1500. c. Chromosome pairing in the diploid showing 16 pairs, 

 permanent acetocarmine. x 1500. d. Chromosome pairing in the triploid hybrid between the 

 diploid and the wild tetraploid showing 16 pairs and 16 univalents, permanent acetocarmine. 

 X 1500. For explanatory diagram see Fig. ^b. e. Chromosome pairing in the wild tetraploid 

 (jV. uniseriatum How. & Mant.) showing 32 pairs, permanent acetocarmine. x 1500. For 

 explanatory diagram see Fig. 4c. /. Chromosome pairing in the autotetraploid of b showing 

 quadrivalents and pairs. For explanatory diagram see Fig. ^d. 



are conveniently designated as JV. officinale R.Br, and N. uniseriatum Howard & Manton, 

 respectively. 



Since JV. uniseriatum* (unlike Spartina Townsendii) has existed for long enough to 



* Some important new facts have recently been added to this subject by H. K. A. Shaw (1947) from a 

 study of the literature and herbarium material which appears to show that the tetraploid species is 

 probably more widespread than had previously been thought since specimens attributed to the new 

 species have recently been reported from as far afield as America, Africa and Afghanistan. The name 



10 



