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GARDEISERS- CHRONICLE 



I Departments of Foreign Exchange and Book Reviews 



„,.i 



ON "HARDENING PLANTS." 



FEW garden practices are so universally followed by good 

 gardeners or so little understood by anybody, as is the prac- 

 tice of hardening plants. Kveryone who raises plants under glass 

 knows that to transfer them directly in the early part of the 

 \ear from the heat in which they are grown to the chances and 

 changes of temperature in the open is to court failure. Garden- 

 ers aiso know that the hardening process is somewhat slow and 

 that one or two days only in a cold frame does not suffice to 

 harden young plants. E.xpenments carried out a few years ago 

 by Mr. Harvey*, show in a most interesting manner that al- 

 though the hardening process begins at once when plants are 

 transferred from greenhouse to cold frame — in one day the plants 

 acquire some measure of increased resistance to cold — yet at least 

 five days elapse before the plants become really hard. Thus Cab- 

 bage, after five days' e.xposure to cold frame temperature — three 

 degrees above freezing — were able to resist thirty minutes' e.x- 

 posure to 3°C., although the temperature froze them stiff and 

 killed outright unhardened plants. He also showed what is of 

 great interest to gardeners in this capricious climate, that hard- 

 ened plants lose under warm conditions their hardiness is about 

 the same time as they acquire it. If, therefore, tempted by a 

 genial spell of weather, the gardener puts out his hardened plants, 

 and if that genial spell lasts long enough to make the plants 

 soft again, they are almost as prone to damage as they would 

 have been had they not been hardened at all. "More haste less 

 speed," is the motto to observe in planting out. Physiologists 

 who have investigated the effects of frost on plants have given 

 us a good definition of hardiness. It is the ability to survive ice 

 formation within the tissues. Of our garden plants the tender 

 ones do not possess this power, and we know of no means of 

 making them acquire it. Some possess it, as it were, naturally ; 

 others, and they arc the ones that interest us here, may acquire 

 hardness. The Cabbage is an example of this last class, whereas 

 the Tomato cannot be "taught" to resist a temperature below 5° 

 Centigrade. The adept in gardening can tell by the appearance 

 and still more by the "feel" of a plant, whether it has been well 

 hardened. This is due to the fact that during the hardening pro- 

 cess growth is checked and hence leaves and soft stems, instead 

 of appearing sappy, have a stiffish appearance and are springy 

 and elastic. They are smaller and thicker than are those of un- 

 hardened plants of equal age. ICxperiment has shown that hard- 

 ening may be assisted in watering plants with solutions which 

 check growth. Give them nitrates such as potassium or calcium 

 nitrate and the plants' growth is stimulated. They become sappy 

 and soft, but water them with a weak solution of common salt 

 or washing soda (at the rate of about two oz. to the gallon) and 

 their hardiness is increased for the same reason that it increases 

 in the cold frame, namely, because growth is checked. Withhold- 

 ing water is. of course, another means of aiding the hardening 

 process. It follows from what has been said, that age is a 

 factor to be taken into account. Young tissues, for example those 

 of leaves, are more easily injured than are older tissues; a fact 

 which has to be borne in mind in Autumn sowing and .Autumn 

 planting. There are apparent exceptions to this rule : for ex- 

 ample, July sown Beet may be left in the ground, if the soil be 

 not a very wet one, well into the Winter, by which time old roots 

 wou!d have perished. This, however, is to be attributed to the 

 fact that the "root" of the mature Heet consists of tissues, the 

 cells of which are on the down grade of life, they arc, as it 

 were, over-mature, and begin to show the reduced resistance of 

 old age. Plant<; which are hardy in the sense in which the word 

 is used here may .show signs of "frost-bite," although ihcy do 

 not succumb to a hard frost. These signs— also exhibited by 

 tender plants when grown in too low a temperature— take the 

 form of spots on the leaves. These spots arc at first translucent, 

 owing to the fact that they represent areas in which the intcr- 

 celhilar spaces, normally full of air, have become injected with 

 water v hirh has been excreted from the neighboring cells as a 

 result of the low temperature. Even hardy plants may show 

 thece frosted areas, but in their case the frost spots disappear, 

 whereas in tender plants they become brown as the cells dis- 

 intcgr.ite Some hardy p'a 'ts, like ihe Cabbage, show curious 

 after-efTccts in the spotted areas. As Mr. Harvey has shown, 

 each snot eivcs rise in the course of a few davs to an embossed 

 area of a color lighter than that of the rest of the leaf. These 



•"HarHtnin* Process in Plants and PevdopmcnK frf-ni Fn.si Injury." 

 Joirtiai of A i^ricutlural Rnrareh, XV., 2, 1918 



intumescences go on growing and may reach a large size, and a 

 Cabbage leaf which has been exposed to and recovered frum frost, 

 may have its surface puckered and rolled in most fantastic shapes. 

 This behavior may be likened to that often produced by me- 

 chanical injury and the formation of those intumescences must be 

 relerred to the plant's reaction to the wound-stimulus of frost, 

 iiiooni on the leaf ofte.i serves, as may be shown by observing 

 different varieties of Cabbage, to prevent injury from frost. This, 

 according to Mr. Harvey, is probably due to the waxy layer act- 

 ing as a water-proof preventing communication between water on 

 the outside of the leaf and that lining the cell walls and occurring 

 in the cells. When frost comes, the water on the leaf-surface 

 is cooled and in the absence of wax this cooling is transmitted 

 to the water of the cell-walls and cells. As the process continues, 

 this water is undercooled. It docs not freeze, however, unless 

 the ice crystals which form on the surface are in continuity with 

 the water in the walls and cells. The bloom breaks the continuity 

 and hence ice — which is the danger — forms less readily in a leaf 

 with bloom than in one wiihout it. There are numerous hy- 

 potheses as to the actual cause of death from freezing ; the 

 most probable is that as water escapes from the cells, the nitrog- 

 enous complex compounds (proteins) on which cell vitality de- 

 pends are salted out, that is, precipitated, and that once this pro- 

 cess has been set up, recovery is impossible, because the vital 

 mechanism has been destroyed. Hardening on this hypothesis is 

 due to a change in the composition of the proteins of the cell. 

 During the process of hardening these substances give rise to 

 other proteins of simpler construction which are less apt to be 

 salted out, that is, thrown out of the vital mechanism. 



It is common practice to spray plants which have been sub- 

 jected to frost. The practice is a good one. but how it achieves 

 the end is obscure. It was thought at one time that it caused a 

 slower thawing and gave time to the cells which had lost water 

 to recover it. In point of fact, spraying hastens the thawing pro- 

 cess and it seems more probable that its good effect is due to 

 transpiration being checked. Mr. Harvey, has made a curious 

 observation which lends some collateral support to this view. He 

 finds that if the leaves of a frost-spotted Cabbage are submerged 

 in water, the intumescences already described do not develop. 

 Water appears, therefore, to arrest the pathological processes set 

 up by frost and the recovery of a frosted leaf which has been 

 sprayed must be included in the already long list of "water 

 cures." — The Gardeners' Chronicle (British). 



PLANT PESTS AND THEIR PREVENTION. 



THE large amount of space in gardening papers devoted to plant 

 pests and remedies for their extermination would seem to 

 indicate that enemies to plant life are becoming a serious menace 

 to horticulture. .Assuming this to be a recognized fact it might 

 not be amiss to try and find causes for this increasingly serious 

 state of things. There arc at the moment many capable men who 

 are engaged in the praiseworthy task of finding remedies for 

 the various troubles which constantly threaten with disaster the 

 efforts of gardeners in the production of healthy fruits and vege- 

 tables. The plan adopted by these painstaking laborers is to 

 work out the life-histories of the various fungi and insects which 

 jirey upon plants, and, having ascertained their weakest and most 

 assailable parts, to provide weapons to overcome and subdue 

 them. The scheme is a perfect one, and gardeners are indebted 

 to them for their labors. But it seems to me, and it will ap- 

 pear equally clear to the reader, that gardeners and cultivators 

 are very much to blame for the increased prevalence of plant foes, 

 and it may be reasonable to suggest that they may do much to 

 back up the efforts of the plant <ioctors by taking increased pre- 

 ventive measures. 



The old saying, "Prevention is better than cure," has become 

 .so much of a platitude that it has lost much of its significance, 

 but the truth of the saying is of such paramount importance to 

 the gardener that he might well make it the leading maxim of his 

 vocation. There are. of course, cir'ain fnimus diseases which 

 occur in the form of epidemics, like the visilations of the dreaded 

 Potato disease, while certain forms of aphis will develop to an 

 alarming degree in time of drought. These attacks, so far as 

 prevention is concerned, are more or less beyond the control of 

 the cultivator, who must needs relv on the aid of the plant pa- 

 thologist lo surccssfully combat them. On the other h.nnd. the 

 increase of manv plant diseases and |x>sts might Ik- considerably 



