20 



r.ARDE!\lERS' CHRONICLE 



Leaf Falls Without Frost Bite 



AJ'TER a night's sharp trost in Auiunm \\ c arc not 

 >urprised when wc take our walk to see the grotnid 

 httered witli fallen leaves from every shade oi yel- 

 low to hues of the deepest red or hrown. In fact, we look 

 expectantly for this sudden spreading of a leafy car])el as 

 a natural sequence to the change of temperature, and quite 

 as a matter of course we attribute it to the work of the 

 nipping frost. 



Before we take things so absolutely for granted let 

 us take a little closer look into the matter and see what 

 really takes place: for. after all, in their own good time 

 surely the leaves would liave fallen, even though the 

 frost had never been. 



Again, in tropical coimtries where rain does not occur 

 for several months at a time plants act in an exactly 

 opposite manner to those in our temperate regions, for 

 their leaves fall before the hot, dry season, while in 

 climates where no lengthy and alternating periods of hot 

 and cold occur the "fall of the leaf is a very inconspicu- 

 ous event, as new leaves form and develop at the same 

 time that the older ones are falling away, so that the trees 

 appear, more or less, to be always in full leaf. Ever- 

 greens, in our own climate, present a somewhat similar 

 case, but even these have to shed their leaves sooner or 

 later, although in some cases they may last for several 

 years. 



It would, therefore, appear that some other potent 

 factor or factors besides cold and frost must he at work 

 at the time of leaf fall. Before we can make any pro- 

 gress toward this end, however, we nuist assimulate one 

 important fact, namely, that the great engine which 

 provides the motive power of all life, both plant and 

 animal, is sunlight. 



Now- the leaves of plants are organs especially con- 

 structed to intercept rays of sunlight : every leaf of every 

 plant is adapted, so far as circumstances and environ- 

 ment have permitted, to present to the sunlight as much 

 surface as possible, hence we get endless forms of leaves 

 on dififerent species of plants, those t'orms fittest for the 

 environment naturally surviving. 



Die Before Tropic Dry Season 



Leaves are continually transpiring watery vapor from 

 the stomata, or openings on their under side ; this trans- 

 piration is, of course, kejit more or less active according 

 to the dr\'ness of the surrounding atmosphere. In very 

 hot weather it is apparent that transpiration would be 

 most active, and this at a time when most inconvenient 

 to the best interests of the plant, for then the soil would 

 be so dry that it could not supply the increased demand 

 made upon it. Hence it obviously follows that if a plant 

 growing in a hot climate could dispose of its leafy 

 raiment before the dry season, and so largely shut in 

 the store of moisture that it then possessed, it would 

 be adopting a most protective measure. Such is the way, 

 then, by which, when the water supply begins to fail, 

 such plants prepare to meet the situation, and, in due 

 course, when the dry season arrives their leaves fall 

 and the so-called "Summer sleep" commences. 



Later in the sea.son, when moisture again impregnates 

 the soil and the water supply within the plant is naturally 

 very Vnv, the mineral salts in the plant tissues greedily 

 absorb any moisture that comes their way. and so the 

 young roots are incited to seek for greater supplies. 



If the "fall of the leaf" before hot, dry periods can be 

 so explained, what of our climate, where the exact op- 

 posite, a long, cold and damp period, occurs. We know 



that the leaves f.all liclnre tlie Winter sct> in. hut that 

 their fall cannot be associated with a lack of moisture 

 is obvious. What, then, is the influence at work in this 

 case ? 



Let us suppose that we have a delicate, sensitive plant 

 growing as a pot plant and that we water the soil in 

 which it grows for a few times with very cold water. 

 .\lthough the surrounding atmosphere may be most fa- 

 vorable to its development and the temperature of the 

 soil well above zero, yet we find that the leaves of our 

 plant begin to flag and soon the whole plant looks as if 

 it had been nipped b}- the frost. What has happened? 

 The very cold water has lowered the temperature of the 

 soil so much that the activity of the roots has been in- 

 terfered with, and as a natural consequence their absorb- 

 ing powers have decreased. But the leaves in the warmer 

 atmosphere above have gone on transpiring ai|ueous 

 vapor as usual: thence the tissues have lost their water 

 and it has not been replaced. So the jilant shrivels up 

 and dies. 



At the sanie time also liy dis]iosing uf their leaves trees 

 avoid other serious dangers. Water in the tender tissues 

 of the leaves during the times of frost would be most 

 dangerous to the plant structure, for then it would be- 

 come ice and with the expansion that takes place when 

 water freezes the tissues would be ruptured. How 

 dangerous such an enemy would be to the delicate vege- 

 table cells can be readilv demonstrated by the bursting 

 of a water pipe when the water contained in it expands 

 in the process of freezing. Another danger, overcome 

 by the absence of leaves, is that of heavy falls of snow. 

 Trees covered with foliage during heavy snowstorms 

 get their branches sadly broken by the weight and the 

 pressure of the wind. 



It should be observed ton. that trees in the lowlands 

 retain their leaves several weeks longer than those of 

 elevated mountain regions, even when of the same species. 

 In the latter situations snow and frost often occur in 

 the Autumn months, and consequently the soil is cooled 

 sooner than that of the lowlands. I'roni this w'C might 

 suppose that these mountain plants would have to put 

 forth their leaves several weeks earlier in the year than 

 their lowland relatives. As a matter of fact, the exact 

 opposite is the case, for the mild temperature of Spring 

 is later in reaching these elevated regions : hence it comes 

 about that in these colder regions those trees survive best 

 which have adapted their season's work to commence a 

 few weeks later and finish a few weeks earlier than the 

 normal period. 



PiTREi.Y Protfxtive \1f..\sitke 



\\'e may reasonably assume that the "fall of leaf" is 

 chiefly a protective measure adopted by the jilant against 

 transpiration at times when it would be injurious to its 

 economy and that it is brought aliout by either heat or 

 cold. .At the first indication in late Sunmier of a lowering 

 temperature some iieculiar layers of cells begin to form 

 at tlie base of the leaf stalks and quicklv extend across 

 the tissues, disconnecting thcni. — .Vrrc ]'(>th Sun. 



Flowers liave an expression of countenance as nuich 

 as men or animals, .'^(lme seem to smile: some have a 

 sad expression : some are |>ensive and diffident : others 

 again are plain, honest, ui>ric;ht, like the br()ad-faccd sun- 

 flower, and the hollvhock. — !Ir)ir\ Ward Bcrcher. 



