MR. PERCY GROOM ON BUD-PROTECTION IN DICOTYLEDONS. 257 
The result of the excision was nil. The experiment merely showed that the hairs are 
not essential to the older leaves. The following points are worthy of note with reference 
to the secretion :—(i.) the secretion, being adhesive, glues all the young parts together ; 
(ii.) forming a thin coat over the young exposed leaves it diminishes transpiration 
by reflecting some of the sun’s rays; (iii.) mucilage, being hygroscopic, diminishes the 
loss of water due to transpiration. In addition it might be suggested that mucilage 
prevents the bud being drained of moisture by excessive transpiration of the older leaves. 
Wiesner states that in the well-known sympodial shoots of dicotyledonous trees the 
death of the growing-points is occasioned by the older leaves draining the apex (14). 
Pfeffer’s measurements of the osmotic power of gum arabic give an idea of the energy 
with which the secretion sucks in water. Attention does not appear to have been called 
to the fact that the employment of a hygroscopic substance like mucilage (and tannin) 
is an admirable means of eontrolling the water-supply of an organ for two reasons: 
first, the osmotic power of a solution increases with a rise of temperature ; secondly, the 
osmotie power inereases with the concentration of the solution. Тһе result is that when 
a bud is in greatest danger of losing all its water—i. e. when the temperature is high and 
a considerable amount of water has been evaporated from the mucilage—the remaining | 
water is held most firmly or a first supply of water is absorbed most fiercely. 
It is probable that this external secretory apparatus is also of use in removing excreta 
from the young cells. The following reasons may be given in support of this view :— 
(i.) the precocious development of internal excretory organs suggests the importance of a 
speedy removal of products which would retard the activity of the young cells; 
(ii.) many plants which possess colleters &e. also have internal structures which manu- 
facture or store up the same substances, resin and mucilage, e. g. Rubiacee, Apocynacee, 
Asclepiadee, Guttifere, Dilleniacee ; (ii.) we are unable to attach any significance 
{о the secretion of mucilage by the leaf-tips of certain submerged water-plants, 
е. g. Myriophyllum, Ceratophyllum (8); (iv.) it is suggestive, too, that laticiferous tubes 
should penetrate the colleters, as І have shown to be the case in the Apocynacee. This 
view of the double nature of the service performed by colleters affords a means of 
conceiving of their primary mode of origin in several distinct unconnected families. 
Originally, then, the young epidermis of the plants became the recipient of excreta. 
The plant soon found the advantage of completely ejecting the excreta, so that deeper 
tissues need not share in their storage or removal: so the excretion oozed through the 
epidermal walls first over the whole surface of the leaf, and subsequently in localized 
places. Тһе plant discovered the benefit of the secretion as a protective agent, and 
thence the external secretory apparatus was elaborated till it reached its present state of 
perfection. Тһе various stages above mentioned actually occur in different plants. Тһе 
whole process may be compared to that by which substances which were originally 
coloured katabolites (tannin, &c.) subsequently became also protective in function. 
Many plants possess other glands which may be protective in function—water-stomata. 
Gardiner (12) and Moll regard these as species of safety-valves to permit the escape of 
excess of water, and thus prevent rupture of the delicate tissue of the young leaves. 
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