Research Bulletin (Iowa Agriculture and Home Economics Experiment Station)


The breeding of hardy apples is naturally such a long time proposition that it would be a distinct advantage if by microscopic examination of the wood, by mechanical tests, or by other observations the hardiness of a tree could be determined while it was still in the nursery, The following is a summary of the facts brought out in the investigation of "Hardiness in the Apple as Correlated with Structure and Composition. "

1. The cutting, compression and penetration tests seemed to indicate some correlation between hardiness and hardness, but exceptions were found.

2, Twigs from northern sections, except the forms of Metlus rivularis, Red June and Patten, were no harder than twigs from Iowa and further south.

3, Specific gravity tests of dry wood showing density corresponded very closely with the mechanical tests showing hardness, indicating that the two tests are fairly accurate means to the same end.

4. Variat ions in the specific gravity of twigs of the same variety from different sources were noted, as was also the case in the mechanical tests. Twigs of the same variety from the same source gave pretty uniform results.

5. The specific gravity of twigs varies, to some extent depending upon what part of the twig is used. The tenderer varieties, which mature a little later in the summer, gave the greatest specific gravity a short distance back of the tip; earlier maturing varieties increased in specific gravity in proportion to the distance from the tip. As the tests were made in July this difference is in all probability due to the difference in the maturing of the twigs used.

6. The maturity of the wood at the time cold weather sets in undoubtedly has a most important bearing upon the ability of the tree to withstand cold.

7. By noting the formation of terminal buds and the time of leaf fall a pretty accurate idea of maturity can be secured, except in case of a few varieties.

8. The hardier varieties on the average had a slightly lower moisture content than the more tender varieties.

9. The difference in water content can be explained partly at least by the fact that the more tender sorts evaporate water more readily than do the hardy varieties. Freezing tends to dry the twig out, and after a period of very cold weather the twigs of the hardy varieties are generally found to contain the most moisture.

10. The rate of evaporation is due in part to small differences in thickness and structure of bark, including differences in the number of cutinized layers.

11. Most of the hardy varieties contained a large amount of starch stored in the pith and medullary rays. The Malus ioensis, however, proved to be an exception in this regard.

12. Large, thick petals are correlated with hardiness, although the converse of this is not always true.

13. The results of the freezing experiments show that within the limits observed a sudden drop in temperature is more injurious than the actual degree of cold.

14. While all twigs not previously dried were injured to some extent when held in a temperature of -10 degrees F. for 20 minutes, the injury was less in the hardier varieties, thus showing their ability better to withstand such a sudden drop in temperature.

15. While in the various lines of comparison which were made there were found many indications of morphological differences between hardy and tender varieties, yet from the practical view point as yet it is impossible to name anyone test by which the degree of constitutional hardiness of a seedling apple may be foretold. Among the various tests for hardiness, that of the length of season required by the tree to mature the season's growth is of first importance. Perhaps by taking careful notes on a number of trees of any particular variety for two or three years or more, noting their time of starting and cessation of growth, their ability to produce a good root system from the cion, their water content, the resistance of their twigs to sudden zero temperatures, and their rate of evaporation, a pretty accurate idea of the ability of the tree to withstand, ,cold might be obtained. If in addition the variety has hard wood, a good amount of stored starch and large petals these would be further indications of hardiness, although from this study it appears that these points are of less importance than those first named.



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