Several external factors may be the cause or occasion of this duplication of chromosomes. A severe frost, for example, will temporarily suspend divisions in the pollen grains and cause some germ-cells to form (provided the frost has not been too severe), bearing twice the normal number. These on fertilising normal egg-cells will produce triploids or, if the egg-cells have been similarly affected, tetraploids. In gardens where many plants are out of their natural environment, having come from countries with more regular climates, such occurrences are not infrequent. In their own country, once the winter is over, they produce their flowers with no set-backs, but in England, where we get a warm spell in the spring long enough to bring out the flowers, often followed by a severe frost for several nights just as the flowers are forming, many aberrations and abnormalities arise. Similarly in a wild state, an unusual season may upset many of the normal mechanisms of the plant. De Mol has discovered that many of the different chromosome types in bulbous plants have been due to the custom of drying off the bulbs after the flowering season and, in the case of bulbs for commerce, the subsequent forcing to produce early-flowering for culture in pots. These bulbs, being subjected to various degrees of temperature at the time of the formation of their germ-cells (these being formed deep down in the bud during the previous summer or autumn before flowering), produce various irregularities of division. Many of these irregularities fail to carry on but the few that struggle through will give rise to new races, sometimes of great beauty, for our gardens. In wheat plants subjected to high and low temperatures at the time of pollen formation Bleier found many grain with twice or even four times the normal number of chromosome. Gametes with twice the number were also produced by Rybin and Eghis in Nicotiana (Tobacco) by chloroforming the shoot and thus suspending the reduction division. The Marechall found that tetraploids could be produced in certain mosses by wounding and from these a few octoploid were produced in the same way. Beyond this, however, it was apparently impossible to go. In many plants and animals X-radiation is a powerful cause of the duplication of the chromosomes.
|Fig. 86. Polyploid gametes in the Hyacinth variety "White Duc Maxima". a, normal pollen grain bearing 12 chromosomes; b, diploid grain with 24 chromosomes; c, tetraploid grain with 48 chromosomes. (After de Mol.)|
|Fig 87. 1, chromosomes of a triploid Hyacinth "King of the Blues" with 24 chromosome (12 long, 6 medium and 6 short); and two aneuploid bud variations which arose from it with 18 and 21 chromosomes respectively (2 and 3). Both variants are dwarf, all organs being considerably smaller, and the indigo colour of the "King of the Blues" has changed into carmine in both case. (After de Mol.)|