582 



BOTANY OF THE LIVING PLANT 



A polyploid organism is one which possesses more than two " sets " of 

 chromosomes in its somatic cells. Polyploids are very common among culti- 

 vated plants. The chromosome complements of some of the cultivated forms 

 of Rubus are shown in Fig. 443. The Raspberry (the variety figured is " Super- 

 lative ") is a normal diploid with seven chromosome pairs (two " sets ") . 



A 

 B 



C 

 D 



a 



B 



D 



a 



B 



H 



b c d e f g 



Fig. 442. 



Diagram illustrating types of chromosome rearrangement. The letters indicate 

 successive segments of the chromosomes, a, normal chromosome ; b, fragmentation ; 

 c, inversion ; d, deletion ; e, a pair of normal chromosomes ; /, reciprocal trans- 

 location of segments C and A, resulting in g, " ring-formation " at meiosis. (From 

 Crane and Lawrence.) 



the " Mahdi " is triploid, with three sets ; the Veitchberry tetr aphid, with 

 four sets ; the Loganberry hexaploid, with six sets ; and the Laxtonberry 

 heptaploid, with seven sets. 



/ 





Or -U 



Superlative2n=l4 Mahdi 2n=2l 



Vfe!tchby2a=28 





1 



*A 





% 



Loganberry 2n =42 Laxtonberry 2n- 49 



Fig. 443. 



Somatic chromosome complements of some diploid and polyploid 

 Rubi. For explanation, see text. (From Crane and Lawrence. After 

 Crane and Darlington.) 



Polyploids can arise in several different ways. For example (1) the chromo- 

 some number of a somatic cell may become doubled (Fig. 444, a, d). This type 

 of polyploidy can be induced artificially in certain plants, e.g. in the Tomato 

 by encouraging the development of adventitious buds from a wound-callus. 



