THE STEM OF YASCULAR PLANTS. 159 



made with but little nicety, for no one has been able to 

 repeat it in the manner mentioned. 



It is to be remarked, as we have before observed, that 

 there is no real interval between the layers ; but that that 

 which appears as such after maceration is nothing but 

 round cellular tissue. Dutrochet appears to me to have 

 proved that this zone of cellular tissue represents for each 

 woody layer, that which is the pith for the central one, 

 and that thus an entire woody body is formed of bodies 

 resembling each other, except the differences caused 

 by their position ; others think that this zone of round 

 or elongated tissue, may be produced by the slow 

 growth during winter ; the cellules would have, accord- 

 ing to them, the time then to become round and 

 developed in every direction; whilst, when the growth 

 is rapid, they will be elongated, and, as it were, dragged 

 along by the increase of the vessels. 



It would result from this manner of considering; the 

 zones which separate the annual layers ; — 1st, That this 

 separation ought to be so much the more perceptible as 

 the alternations of the vegetation (those which are caused 

 either by the fall of the leaves or by the alternation of 

 the seasons) are more distinct ; this is not observed in 

 comparing trees of the North and the South. 2d, That 

 an accidental stagnation of the vegetation caused, during 

 summer, by a return of cold, or by any other cause, 

 ought to produce a cellular zone analogous to that of 

 the ^vinter ; just as a very mild winter might cause the 

 annual zone almost to disappear. Hill asserts that, in 

 many cases, two distinct layers are formed during the 

 year ; the one owing to the sap of the spring, the other 

 to that of the summer : he calls these the layers of the 

 seasons. Adanson indicates, on the contrary, that the 

 layers of certain years are often mingled together, from 

 observing that Elms of a hundred years old, cut down 



