388 M. Ehrenberg's Observations on Microscopic Organisms, 



on the margin of the Nile, existed in such vast abundance, 

 that without going the length of asserting that they absolutely 

 predominate, still it is a fact, that there is not a particle of 

 this soil of the size of half a pin's head in which, making no 

 allowance for the chemical changes which may have taken 

 place, there was not one and frequently many of these animals. 

 We may now, therefore, safely affirm, that the deposits in 

 harbours, and even the accumulation and the extraordinary 

 fertility of the mud of the Nile, and probably of all other river 

 deposits, proceed not solely from the gradual destruction and 

 mechanical transport of one portion of solid soil to the forma- 

 tion of another, no more than that they are solely the product 

 of the vegetation of plants ; but, on the contrary, that they 

 result from the immensely rapid agency, hitherto scarcely re- 

 cognised as vital, of animal organisms, which are undiscernible 

 to the naked eye, but whose quantitative and natural limits 

 must henceforward be inquired into, and which, from this 

 time, must be considered as possessing a very important in- 

 fluence upon these natural phenomena. 



Vegetable Fhysiology. 

 Cause of the concentric rings in trees. — Theory leads to the 

 presumption that in those countries which are uniformly warm 

 and moist, we should not be able to calculate the ages of trees 

 by the same rule as in our temperate climates, in other words, 

 by the number of concentric ligneous rings, or layers. In fact, 

 the arrangement of the wood of our forest trees in layers has 

 always been considered as produced by an interruption in the 

 formation of their tissue, an interruption which is caused by 

 the reign of winter in these colder countries. It is probable 

 that the excessive droughts which lead to the fall of the leaf 

 in the trees of some countries, such as the interior of Brazil, 

 in Senegal and Egypt, produce a somewhat analogous effect. 

 But, on the other hand, in the forests of intertropical regions, 

 where heat and humidity invariably predominate, the growth 

 of trees ought to be sensibly regular throughout the whole of 

 the year. This has been observed in the coesalpinia and other 

 dye-woods. It is desirable that individuals who are favourably 

 circumstanced for making observations of this nature, would 



