Olo 



EXOGENS. 



EXOGEXS. 



The sinuosity and partial obliteration of the medullary processes is 

 ft principal cause of the anomalous appearances at figs. D, E, F, G, 

 where they are reduced to fine lines, only visible beneath a micro- 

 scope, and not radiating from the centre, but disposed in no certain 

 manner, sometimes even transversely, owing to the excessive disturb- 

 ance of the wood itself. In jig. I, the singularity of structure is 

 owing in part to the excessive irregularity with which the wood has 

 been developed, and in part to the looseness and irregular shape of 

 the medullary rays, which seem huddled as it were round the woody 

 cords; the latter are moreover extremely variable in size, some of 

 them being as much as half an inch in diameter, and others so small 

 as to consist of no more than a single vessel with its usual coating of 

 woody tissue. 



The absence of concentric circles is an extremely frequent occur- 

 rence in the wood of tropical countries, and it is almost certain that 

 many families of Exogens never form them visibly under any 

 circumstances. We say visibly, because in fact they must be annually 

 formed in all cases, although we do not see them. The reason why 

 Exogens have their wood marked by concentric circles is, that the 

 ligneous tissue formed at the end of a season is more compact than 

 that formed at the beginning, and hence, as the two are in juxta- 

 position, the difference in their density distinctly separates the one 

 from the other. But if, from any cause, whether proper to plants 

 as species, or owing to the external influence of an equable climate 

 the tissue of wood formed at all seasons is exactly alike, no zone will 

 be visible, although in fact the formation of the wood is exogenous in 

 the most regular manner. Such cases are seen at jigs. K, L, S, and 

 elsewhere in the illustrations of the present article. 



It is not a little remarkable however that while the wood in some 

 cases has no trace of zones, the bark should show them most dis- 

 tinctly, as in the instance offg. K. 



1C 



When a deep zone of cellular substance is formed between each 

 zone of wood, a curious banded appearance is produced, as in the 

 singular Indian climber marked N M O P Q, where extremely excen- 

 trical growth in combined with this peculiarity. At N we have the 



stem two years old, the second zone passing pretty regularly round 

 the first and cut oif from it by a broad deep band. At M the specimen 

 is of the same age, but the second zone is formed on one side only. 

 At the specimen is two years old, with the first zone perfect, but 

 the second broken up into a number of unequal irregular pieces, and it 

 would seem as if a third growth had commenced on one side (to the left 

 of the cut). At P the growth is of three zones, both the second and 

 third being much lobed, and the third only extending three quarters 

 round the second. Finally, at Q, where the irregularity is the 

 greatest, there is a growth of four zones, the first symmetrical, the 

 second very much deeper on one side than the other, the third but 

 half surrounding the second, and the fourth formed only along two 

 ridges on the third. 



If it happens that, in addition to the presence of a thick cellular 

 layer between each zone, the medullary processess are also very 

 thick, an appearance still different from the last is produced, as 

 at Jiy. R. 



R 



That wood is sometimes formed in the bark itself has been long 

 since shown by Mirbel, in the case of Calycanthws Jtoridus, where four 

 additional woody columns appear equi-distant in the bark, without 



any separate pith, but radiating from their first line of origin. We ai o 

 now acquainted with many such cases. In fig. T are the commence- 

 ment of four such columns at a on one side ; but in that specimen no 

 further indication of such a structure is visible; but at fig. U, which 



