140 



SCIENCE-GOSSIP. 



THE BIRCH AND THE ALDER. 



By P. Q. Keegan, LL.D. 



'"pHE natural order Betulaceae includes only the 

 two genera Betiila and Alnus, and these are 

 taxonomically distinguished by some minor dif- 

 ferences, with respect to the texture and durability 

 of the scales of the female catkins. In the birch 

 (Betula alba) the bark is smooth, silvery white, 

 and scales off in transverse chips ; the leaves are 

 deltoid or rhomboid, and irregularly serrate ; there 

 are two folioles around the male flowers, and the 

 fruit has broad membranous lateral wings. In the 

 alder {Alnns glutinosa) the bark is dark brown and 

 seamed with clefts ; the leaves, when young, are 

 obovate, blunt, wavy, serrate, and glutinous ; the 

 anthers are bilocular, and there are four folioles 

 around the male flowers ; the fruit is hardly 

 winged. The main difference is, that in the birch 

 the female catkins have thin, deciduous scales, 

 while those of the alder are thick, persistent, and 

 woody. As regards this special feature, it may be 

 remarked that the scales are persistent, because 

 they become woody, while the thin scales Avhich 

 never lignify are deciduous. Perhaps this \-ery cir- 

 cumstance, trifling as regards morphology and 

 taxonomy, may furnish the key to immense and 

 very important differences in respect to chemistry 

 and physiology. The object of the present paper 

 is to endeavour to elucidate these relationships. 



First, it will be advisable to inquire if the in- 

 ternal anatomy and histology of these arboreal 

 organisms present similarities corresponding with 

 the external morphology. The characters of the 

 xylem of the birch are as follows : The vessels 

 are very numerous, and either stand isolated or in 

 a radial file, of rarely more than two or three to- 

 gether. The Avood parenchyma is scanty, but 

 occurs either in isolated cells or in very straight 

 transverse bands. The fibres are radially disposed, 

 and have a thick uncoloured wall. The medul- 

 lary rays are numerous, straight, and consist mostly 

 oi one layer, sometimes two, of rounded or oval 

 -cells, which tangentially appear as pointed 

 spindles. The limits of the annual rings of wood 

 are clear, being formed of four or five rows of very 

 flattened cells. The xylem of the alder differs 

 from this only, in the following particulars. The 

 vessels sometimes occur in radial files of as many 

 as four together. The walls of the wood-fibres are 

 not so thick, and the medullary rays are more 

 numerous, crowded, and of a brown colour. With 

 regard to the bast layers, a few bast-fibres are pro- 

 duced in both trees, in the first year, but none 

 afterwards ; hence the secondary bast from the 

 second year onwards is composed of soft bast only. 

 A considerable sclarification of the liber elements 

 occurs, especially in the birch, where the soft bast 

 is composed of sieve-tubes and of parenchyma 



cells. These describe concentric zones of a certain 

 tliickness, dividing with the medullary rays which 

 are cut vertically or obliquelv, the liber into rect- 

 angles or parallelograms. When these liberian 

 zones have attained a certain age, the walls of their 

 elements gradually thicken till their cavities are 

 completely lost. These highly-Iignified, but not 

 suberified, walls then become pierced by a great 

 number of pits, which mark the exterior orifices of 

 so mani' canaliculi, radiating straight through their 

 now amalgamated mass. At the same time, some 

 of these modified cells are seen to be filled or inlaid 

 with large crystals of oxalate of calcium. The 

 whole tissue of parenchyma and sieve-tubes, how- 

 ever, is not entirely thus transformed. Fre- 

 quently an area of active and living liberian tissue 

 still arranged in concentric zones, remains behind, 

 enclosed between the thick sclerified masses. A 

 series of sieve-tubes remain intact in close 

 proximitj' to the cambium, and the small medul- 

 lary rays are quite unaffected by the extensive pro- 

 cesses of lignification going on in their vicinity. 

 The suber of the birch consists of alternate layers 

 of thin-walled cells invested with a white resin and 

 full of air ; also of thick -walled tabular cells, which 

 form in rapid succession. In the periderm of the 

 alder the cells are thick-walled and of more uniform 

 shape, the inner laver being composed of wavy- 

 walled, more cubical, clear elements, while the ex- 

 ternal portions are early invaded by a homogeneous 

 dark brown mass of phlobaphene, whereby the for- 

 mation of rhytidome is more easily developed than 

 in the allied species. 



Neither of these trees has a tap-root, but both 

 possess strong side roots penetrating to one fool 

 in the case of the birch, and to about two feet in 

 that of the alder. The latter requires more 

 moisture, both of soil and of atmosphere, but is 

 more liable to suffer from frost than the former. 

 The evaporation per square foot of leaf surface has 

 been calculated as 0.050 and 0.009 ^°^ alder and 

 birch respectively. The alder wood, when green, 

 is more easily worked than any other timber ; it 

 is not so durable in air as that of birch, but it lasts 

 much longer in water. The mean specific gravity 

 of its fresh wood is about 0.901, of the dried wood 

 0.551. It contains 41.6 per cent, of moisture when 

 fresh cut, and is composed of 49.1 per cent, carbon, 

 6.r hydrogen, and 44.8 oxygen; the amount of 

 cellulose is reckoned as 54.62 per cent., with 3r.33 

 incrusting matter. The mean specific gravitv of 

 fresh birchwood is 0.919, of the dried wood 0.664. 

 It contains 30.8 per cent, moisture, and its ultimate 

 composition is practically the same as that of alder, 

 but the percentage of cellulose is 55.52, with onlv 

 28.21 incrusting matter. 



