492 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 63 



one or both surfaces, as is well illustrated by some species of the 

 heather family (pi. 12). These are all xeromorphic features, wliich 

 presumably reduce water loss, that one is accustomed to seeing in 

 plants of very arid environments. The inflorescence tends also to be 

 reduced to one or a few flowers, which however may be quite large, 

 as already discussed. As a result of much abbreviated stems, the stem 

 leaves are often highly compacted, sometimes very distinctively so 

 as in the purple saxifrage (pi. 5, fig. 1) . 



POLYPLOIDY AND APOMIXIS 



Typically, the vegetative cells of plants, as the somatic cells of 

 animals, have two sets of chromosomes, a paternal set and a maternal 

 set. This is the well-known diploid condition. But plants, unlike 

 animals, often have one or more additional sets (usually pairs of 

 additional sets) per cell in a condition known as polyploidy. These 

 plants are called polyploids in general, but they may be referred to 

 more specifically as triploids (3 sets), tetraploids (4 sets), etc. 



Early cytological studies of certain European and Scandinavian 

 floras led to the generalization that polyploidy is more common at 

 high latitudes, i.e., arctic latitudes, than at low latitudes. The corol- 

 lary was quickly proposed that polyploids must therefore be more 

 hardy and successful than diploids in the extreme arctic type of 

 environment, hence more tolerant of extreme conditions in general. 

 This conclusion is still generally accepted, but the evidence is not 

 as unambiguous as first seemed. If polyploids were really more 

 tolerant of extreme conditions, then one would expect to find high 

 percentages of them also in alpine areas, arid regions, marine habitats, 

 and disturbed situations, but this is not altogether true. However, 

 much more research on this question is still needed. 



If we subject the proposition that polyploidy is relatively more 

 frequent in the northern latitudes to examination by case study, we 

 find indeed that examples can be found of species pairs (one diploid 

 and one polyploid species) in which the diploid member of the pair 

 has the more southern range (e.g., in genera EmpetruTn, crowberry, 

 and Clethra) . However, the converse can also be found ( Vaccinium, 

 blueberries and cranberries, /m, and Campanula., harebells and bell- 

 flowers). Campanula rotundifolia L. (harebell), for instance, is 

 a widespread circumpolar species, comprised both of diploid and 

 tetraploid biotypes, but almost all known diploid biotypes have been 

 found in Greenland (Bocher). Valid general comparisons with 

 tropical angiosperm floras really can't be made at present, owing to 

 our sketchy cytological knowledge of these floras. Manton studied 

 the fern flora of Ceylon and concluded that polyploidy among Cey- 

 lonese ferns must be at least as great as among British ferns. 



