11 



continues, the plant will become unhealthy, and perish if not relieved." 

 [I may here remark that this frequently occurs in Queensland. 

 Q-rowers find soft-wooded plants, such as pelargoniums, &c., passion 

 vines, and even the pineapple, especially in hot wet weather 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 circula- 

 tion 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.B.'] 

 " 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. 



"We 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 and 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 the 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, includipg 

 nitrates, but for its supply of gaseous food it is mainly 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 oflice of 

 the leaves consists in the interchange of gases. The air contains a 

 quantity of carbonic acid gas (a compound of carbon with oxygen), 

 and upon tiiis, with ammonia (consisting of nitrogen and hydrogen) 

 and water (oxygen and hydrogen), the plant feeds. In day- 

 light, when the leaves ■ are exposed to the sun, they are engaged 

 in imbibing the carbonic acid gas from the air, and in utilising 

 it. They store up the carbon, which is needed for their tissues 

 and secretions, and they set free the oxygen into the air. In 

 this manner, plants, while engaged, in feeding by means of their 

 leaves, act in an opposite way to animals. The latter, when breathing, 

 avail themselves of the oxygen of the air, and give out from their 



