THE CYTOGENETICS OF AUTOHETEROPLOID PLANTS 



357 



of the "gigas" forms of Oenothera. Often diploids, triploids, and tetra- 

 ploids are of about the same size, but monoploids are almost always 

 noticeably smaller. Tetraploid species may even 

 be smaller than the related diploid species, as in 

 Plantago and Alyssum. Silene ciliata has 4n and 16n 

 races which are practically alike in appearance. 

 In a chromosomal chimera the portions with differ- 

 ent chromosome numbers may differ in size (Fig. 

 204). In some cases gigantism is associated with 

 an increase in the size of the chromosomes but 

 not in their number; this is true of Primula sinensis 

 and Phragmites communis. In moss gametophytes, 

 diploids tend to be larger than monoploids, and 

 tetraploids larger than diploids. Here the increase 

 in plant size is a compromise between a decrease 

 in the number of cells and an increase in their 

 volume, which tends to be proportional to the 

 number of chromosomes. In polyploid tulips the heads borne on mono- 

 surface area of the pollen grain and of its nucleus P^oid individuals of 



p , , i • 11 -.1 ji 1 Crepis capillaris. Top: 



is found to vary proportionally with the number monoploid head. Mid- 

 of chromosome sets. The monoploid, diploid, and die: diploid head. Bot- 



, , 1 . 1 11 -1 1 ,- er, ni-r tom: chimeral head, with 



tetraploid pollen grams show a volume ratio oi o : 27 : monoploid and diploid 



64 (Fig. 205).^" sectors. (After HolHngs- 



The histological structure of heteroploids usually 

 differs in certain respects from that of the corresponding diploids. In 



Fig. 205. — Microspores of Tulipa suaveolens with 12 (monoploid), 24 (diploid), and 48 

 (tetraploid) chromosomes. (After de Mol, 1928d.) 



Solanum the cells of the mesophyll, stomates, hairs, and vascular bundles 

 are all larger in the tetraploids, and the leaves are greener because of the 



1^ Sinoto (1925) on Plantago, Jaretzky (1928a) on Alyssum, Blackburn (1927a) on 

 Silene, Gregory (1909, 1914) on Primula, Tischler (1918) on Phragmites, von Wettstein 

 (1923 et seq.) and Schratz (1924) on mosses, de Mol (1928d) on Tulipa, de Mol (19326) 

 on Narcissus. 



