CHAMBERS'S INFORMATION FOR THE PEOPLE. 



the preceding layers, but inside the bark. This 

 kind of stem is therefore an ' outward-grower,' its 

 thickness being yearly increased by the addition 

 of successive concentric layers of wood. 



2. The Endogenous, or Inward-growing Stem, 

 whose increase takes place by the formation of 

 new tissues towards the centre, so that the stem 

 increases in thickness by the newly formed woody 

 matter pushing out that previously formed, and 

 not by the addition of new layers to the circum- 

 ference. 



3. The A erogenous, or Summit-growing Stem, 

 whose increase takes place principally at the sum- 

 mit, by the union of the bases of the leaves. 



These three modifications of stems accord with 

 other important structural peculiarities. Plants 

 having exogenous stems have also an embryo with 

 two cotyledons, or seed-leaves, and their proper 

 leaves have usually a reticulated or netted venation ; 

 all those having endogenous stems have an em- 

 bryo with one cotyledon, and their leaves usually 

 have parallel venation ; acrogenous plants, again, 

 have no proper seed containing an embryo, being 

 propagated by unicellular bodies called spores. 



All these three classes include a certain number 

 of woody stems that is, stems whose tissue is 

 sufficiently dense to form timber ; but that of 

 acrogenous stems (tree-ferns, &c.) is not profitably 

 available for this purpose. Our timber-trees are 

 therefore, in practice, reduced to the first and 

 second classes. But we further find that endoge- 

 nous stems, in their woody form (palms, &c.), are 

 confined to warm regions of the globe, so that these 

 also do not concern the British forester. On the 

 other hand, woody exogenous stems are common 

 in northern countries, and to this class belong all 

 our timber-trees in the British Islands. 



In common language, the trunk is often named 

 the bole; and it is this part which affords the 

 timber for which most trees are reared. The trunk, 

 and also the branches, are covered with bark, 

 consisting of a series of thin layers ; while on the 

 outside of all is a very thin layer of a different 

 substance, called the epidermis, or cuticle. The 

 inner layer of bark receives the name of liber; it 

 was on this substance that the ancients, before the 

 invention of printing, were accustomed to write. 

 Within the bark is the wood, consisting chiefly 

 of wood-cells or tubes and vascular tissues, closely 

 interlaced. In the centre of the trunk is a small 

 space filled with a soft substance called pith, from 

 which proceed the medullary rays. 



The growth of a true exogenous bole is as 

 follows : The stem of a seedling consists at first 

 only of cellular tissue, surrounded by an epider- 

 mis ; but as soon as the leaves have expanded, 

 some bundles of woody tubes are deposited, so as 

 to have the appearance, in cross- section, of a 

 dotted circle just within the skin. As the tree 

 advances in growth, the cellular tissue in the 

 centre becomes the pith ; and rays of cells, forming 

 the medullary rays, extend to the epidermis 

 between the bundles of woody tissue. A mem- 

 brane, or rather layer of vascular tissue, composed 

 of spiral vessels, then forms round the pith or 

 medulla, so as to separate it from the bundles of 

 woody tissue, and the pith, in some cases, takes 

 the form of a star with rays diverging from a 

 centre. In the second year of a tree's life, the rays 

 and the central pith both contract as fresh layers 

 of woody or ligneous tissue are deposited. This 



591 



process of growth of the stem by concentric 

 layers of woody matter is continued every year till 

 the tree is full grown. The medullary rays 

 become changed in time into thin hard plates, 

 which still radiate from the centre to the outer 

 circumference of the tree, and form what is called 

 by carpenters the silver grain of wood. The cen- 

 tral pith in the meantime has diminished to a 

 mere speck in the middle of the tree. The newly 

 deposited layer of wood appears soft and white for 

 the first year, and is called the sap-wood. The 

 inner layers form what is called the heart-wood or 

 duramen, which is generally hard and durable. 

 I As the layers of wood are thus distinct, and as one 

 is deposited, in temperate climates, every year, the 

 age of a tree may be ascertained by counting the 

 number of concentric circles ; but this rule does 

 not always hold good. The sap-wood of regularly 

 formed wood is usually white ; but the heart-wood 

 in many cases changes its colour to brown of 

 various shades, dark red, or even black, according 

 to the character of the tree. The branches pre- 

 cisely resemble the trunk in eveiy feature of 

 structure. 



Decandolle observes that the method of reckon- 

 ing the age of an exogenous stem is not liable to 

 much error, but the inspection must be conducted 

 with the greatest care, for the older circles become 

 condensed into a mass, and their number can only 

 be guessed at by measurement. His plan is as 

 follows : ' When I have got a section of an old 

 tree on which I can see the circles, I place a sheet 

 of paper upon it, extending from the centre to the 

 circumference. On this paper I mark every circle, 

 shewing also the situation of the pith, the bark, the 

 name of the tree, the country where it grew, and 

 any other necessary observations. I also mark in 

 a stronger manner the lines which indicate every 

 ten years, and thus I measure their growth at ten 

 years' intervals. Measuring from centre to circum- 

 ference gives me the circles ; doubling this, I have 

 the diameter ; and multiplying by six, I have the 

 circumference.' He then presents a table of the 

 periods of increase in the diameter of various trees, 

 an inspection of which proves that every tree, 

 after having grown rapidly when young, seems at 

 a certain age to take a regular march of growth, 

 and also that as trees advance in age, they still 

 continue to form layers as thick as they previously 

 did subsequently to the period of rapid growth. If 

 such tables were multiplied to a sufficient extent, 

 they would form data from which, by ascertaining 

 the circumference of a tree, its age might be known 

 without having recourse to the destructive process 

 of cutting deep into the growing timber. ' If one 

 cannot get a transverse section of a trunk, then 

 one must seek for old specimens of each kind the 

 date of whose planting is known, measure their 

 circumference, deduce their average growth, and 

 calculate from them the age of other trees of the 

 same kind, always keeping in mind that young 

 trees grow faster than old ones.' Decandolle cites 

 numerous instances of trees whose ages have been 

 ascertained according to the rule here laid down. 

 Some of these appear to be many centuries, if not 

 thousands of years old ; and what is remarkable, 

 still exhibit symptoms of verdure and vitality. 

 Such calculations are apt, however, to lead to 

 erroneous results when applied to trees grown in 

 warm climates. 



The fact that trees of such vast age continue to 



