25 



they have been introduced, it has been ascertained that they grow at a greater rate 

 than in their native land, and it is supposed that is because the rainfall in Cali- 

 fornia is fairly well spread over the year. Jarrah, karri, marri, blaekbutt, yate, 

 tuart, and certain other varieties of eucalypts are found only in the South- West 

 part of Western Australia, that is, in the region where the rainfall is greatest. 

 Some other varieties — wandoo for instance — are abundant in the South-West, but 

 they are also found in the districts to the eastward, where the rainfall is much 

 less. On the Eastern Goldhelds, where the rainfall is very scanty, salmon gum and 

 gimlet grow to a considerable size, producing timber that is of great use in mining 

 and for other local purposes. Certain of the eucalypts have developed a marked 

 resistance to extremes of heat and cold. Baron von Mueller mentions that one 

 variety of eucalypt — the eoolibah of tropical Western Australia, the Northern 

 Territory, and Queensland — withstands unseorched a frequent heat of 156° Fahr. 

 in Central Australia, nor is this tree affected by exceedingly severe frosts (18° F.). 

 Soil and climate are among the determining factors in tree growth, but there are 

 some varieties of trees which flourish well under conditions that are widely diver- 

 gent from the normal. 



CHAPTER VI. 



UNDERGROWTH AND HUMUS. 



Trees are not the only members of the vegetable world that grow in forests. 

 One cannot enter a forest without at once being attracted by the undergrowth, 

 consisting of shrubs and herbs of many kinds that are growing side by side with 

 the trees. Another thing that one notices is that the ground is covered with a soft 

 carpet of leaves and small branches that have dropped from the trees, and this 

 carpet, is thick or thin according to the class of trees in the forest. When this 

 decays, it forms the rich leaf-mould known as humus. It will be readily seen that 

 deciduous trees (those which shed their leaves annually) must form a thicker humus 

 than those trees that do not so lose their leaves every year. Both the undergrowth 

 and the humus are essential to the healthy life of a forest, and anything that des- 

 troys or reduces it below its natural quantity is an enemy of the forest. It is the 

 humus which provides much of the food of the trees. In the course of time, through 

 the action of the sun and rain and wind, it is gradually decomposed and mixed 

 with the upper layers of the soil and becomes available as plant food. The con- 

 tinual decomposition of humus forms several important products. In the first 

 place, the soil receives all the mineral matter contained in the humus; secondly, 

 large quantities of carbon dioxide are produced, which accelerates the decomposi- 

 tion of the mineral part of the soil rendering it soluble; finally the decomposing 

 humus renders many important substances such as potash, magnesia, and lime 

 soluble in water, so that ihej become available for the use of the tree. So it will 

 be seen that humus plays an important role in the healthy growth of a forest. Un- 

 dergrowth has also a special value. It sheds leaves, which add to the stock of debris 

 that falls from the trees, and when the undergrowth dies off, it decomposes, and 

 adds to the amount of food available for the production of essential nourishment 

 for the tree. When a fire passes through a forest, even when it may do apparently 

 little damage to the trees, it may destroy part or all of the humus, and thus indir- 

 ectly cause serious damage to forest growth. 



