5S 



HARDWICO'S SCIENCE-GOSSIP. 



greens and other plants, as the cabbage and meadow- 

 grass, which maintain their verdure at such times ? 

 However, since there cannot in anything well be 

 effect without cause, let us seek what ground they 

 afford us of offering an explanation of such a differ- 

 ence. In doing this we shall not have occasion to 

 look very far before we shall find, in the examination 

 of the leaves of the first, and I might add, main 

 class of plants, i.e. the evergreens, something 

 tangible to theorize upon. For regarding the leaves 

 of plants — as is generally conceded — as the 

 lungs, through which the funotions of respiration 

 and exhalation take place, we shall not be long, 

 I think, before arriving at something like a definite 

 conclusion on the matter. 



In the leaves of evergreens, or as I will now call 

 them for sake of convenience, cold- adapted and heat- 

 adapted plants, there has been made a provision 

 against too rapid exhalation and inspiration, through 

 which they become naturally adapted for both con- 

 ditions — either of heat or cold. Hence, bearing 

 in mind what has just been stated in reference to 

 the relation which the chlorophyl sustains to light, 

 warmth, and sap, and also of how this latter stands 

 affected to cold, there will be perceived the 

 feasibility of the conclusions I am here seeking to 

 establish ; and to show that, as in cold-adapted 

 plants so in tropical, the same conditions are neces- 

 sary to meet either extreme, I need only mention 

 the fact that were it not so, they would not be 

 fitted to protect the sap against the too detrimental 

 effects of extreme cold on the one hand, or of heat 

 on the other, because the sap in plants, as in animals, 

 is the life; and consequently connected with the 

 presence or absence of which, in part or whole, will 

 be the corresponding issues — life or death, of that 

 part or whole. 



The nature of the protection of the leaves of these 

 plants, as contrasted with those which are so readily 

 and lengthily kept denuded, will be perceived to 

 consist in their clothing membrane or epidermis 

 being formed, in point of texture, much thicker and 

 stouter. It of course is this increased thickness 

 and hardened texture of their epidermis which give 

 the leathery or woody leaves of evergreens their 

 peculiar appearance and consistence, enabling 

 them to withstand external influences of heat and 

 cold so long. So in respect of young shoots; these, 

 when they emerge from the bud, are covered by a 

 delicate epidermis, by which they are enabled -to 

 retain their green colour and succulent condition for 

 some time. Seeing then that the epidermis is so 

 specially designed to prevent a too rapid evapora. 

 tion of fluid matters from the tissues beneath, so 

 must it follow that, according to the nature of the 

 epidermis will its adaptability or non-adaptability 

 be in protecting the sap ; just in proportion to 

 which being protected, will the greenness and vitality, 

 or otherwise, of the leaves be made manifest. 



It might be asked, what can be advanced to 



account for plants in whose foliage the same con- 

 ditions are wauting, as in evergreens, cabbage, and 

 some of the grasses ? To explain this, I regard it 

 as of primary importance that the same facts be 

 still borne in mind with which I started, coupling 

 them with considerations such as these : — (1) The 

 relative difference between the temperature of 

 the earth and that of the atmosphere; (2) the close 

 connection existing, as a rule, in all such instances 

 (for be it here remembered these plants are mostly 

 acauliferous) between underground stem and leaves 

 of these plants. Eor such cases, where we have 

 the sap laid up in a part so protected as an under- 

 ground stem, from which the leaves do so imme- 

 diately spring, can scarcely fail but to maintain a 

 sufficiently vigorous vitality in the leaves them- 

 selves. 



Upon an almost similar principle is it that I 

 account for the reason of petiolate or stalked leaves 

 falling more readily than those that are sessile— the 

 two cases being almost parallel. Of course, viewing, 

 as must be done for the sake of analogy, that the 

 woody stem to which such a leaf is attached, and 

 its petiole, as each severally corresponding to the 

 protecting nature of the earth on the one hand, and 

 the exposed stalk to the subterranean and conse- 

 quently protected stem of the other, it will be per- 

 ceived how it stands to sense why such a leaf should 

 sooner defoliate than the other, where no such 

 inter-appendage as a leaf-stalk divides the two, 

 because the petiole cannot be regarded as a whit 

 hardier than the leaf itself, since both, in point of 

 system, are similarly constituted. 



I do not intend that what I have just said should 

 apply to others than perennials, that is, plants that 

 live for many years, and not of those which by 

 nature are limited to a fixed term of one or two 

 years. But even connected with the fruiting and 

 defoliation of these, and indeed all of them, there is 

 something to be learnt. Eor is it not notable that 

 defoliation in no case precedes fruiting, but succeeds 

 it ? And why? I think the answer is not far to seek, 

 even to one very moderately qualified in the rudi- 

 ments of botanical science. Is not the period of 

 fruiting one in which the secretions are more at- 

 tracted, and perhaps altogether in the case of 

 annuals and biennials, to the fruit ?— hence most 

 probably their death. Whereas in the case of 

 others this process of developing and maturing their 

 fruits would appear to be only so far exhaustive of 

 the energies of the plant as to partly deprive the 

 leaves of their vitality ; and thus aiding their de. 

 foliation, — a theory, by the way, which seems to be 

 well substantiated in the case of the holly, which is 

 known uot to shed its leaves till early in the spring, 

 directly after the berries have ripened. Now there 

 is, I kuow, a very common, though notwithstand- 

 ing, erroneous impression among some people that 

 the holly never sheds it leaves at all ; but this 

 no doubt arises from the fact of its never being 



