27 
extent under favourable circumstances, varying in amount according 
to the pressure, temperature, and moisture of the atmosphere, the 
quantity absorbed by the roots, and the structure of the leaf itself. 
There are thus an influx through the root, an upward current through 
the stem, aud an outflow from the leaves. All these act and re-act 
one on the other j the circumstances that favour the one for the most 
part influence the others. If the one or the other be in excess, the 
plant suffers. If the outflow from the leaves be greater than the 
influx from the root, the plant withers, and unless the balance be 
restored it will die. If the outflow be stopped while the influx 
continues, the plant will become unhealthy, and perish if not relieved.” 
[I may here remark that this frequently occurs in Queensland. 
Growers find soft-wooded plants, such as pelargoniums, &c., passion 
vines, and even the pineapple, especially in hot wet weatlier after a spell 
of dry time, rot at the roots and base of the stems. The cause of this 
is that the roots have absorbed a far greater quantity of water than 
the plants can utilise ; the cells become overcharged, the circulation 
is interfered with, and death ensues by what may be termed vegetable 
dropsy. I might digress still further and venture an opinion that to 
this overcharging of the root cells may be attributed that long- 
standing puzzle ‘‘The Australian dead forests.” — F.M.BJ] The 
upward current is facilitated by the swaying movement of the trunk 
and branches caused by the wind, the alterations of pressure and 
relaxations on the cells and vessels tending to squeeze the sap upwards, 
as shown by Mr. Herbert: Spencer. Capillary attraction, or that 
process by which fluids in contact with fine tubes rise in or between 
them, as the oil rises between the threads of a lamp wick, may also help 
to account for the rise of the sap in plants, but is probably less potent 
than the other causes just mentioned. 
“ AVe have now traced the current of sap from the root to the 
leaf, and in so doing have necessarily adverted to some of the principal 
duties fulfilled by the root, stem, and leaf. The leaves, however, are 
not merely concerned in the evaporation of water ; they have, as both 
feeding and breathing organs, other very important duties to perform 
connected with the absorption and emission of gases. The skin of the 
leaf, especially on its lower surface, is perforated here aud there by 
small breathing holes, or stomata^ which contract or open, according to 
the more or less moist state of the atmosphere, and, perhaps, the 
intensity of the light. Through these pores liquids and gases enter 
and escape. 
“ It is found by chemical research that the greater part of a plant 
consists of carbon and water, to which are added sundry mineral 
ingredients, and others containing nitrogen, the latter element playing 
an important part in the protoplasm and in the albuminoid contents 
of the cells. As we have seen, the plant derives some of these 
ingredients from tlie soil by means of its roots ; it can, for instance, 
procure by their aid water, certain gases' — including carbonic acid gas 
and ammonia — various mineral ingredients and salts, including 
nitrates, but for its supply of gaseous food it is mainl3'- dependent on 
the leaves. These organs not only allow of the outflow of water, but 
they drink it in under certain circumstances like the roots. This is 
shown by the manner in which a withered plant regains its firmness 
when syringed. Still it is probable that the most important o£B.ce of 
