166 VEGETABLE ORGANOGRAPHY. 



their extent. They form the reddish spots which are 

 seen upon planks of Beech or Oak cut obliquely. It is 

 in this manner that is made what workmen call Dutch 

 Oak, which was formerly thought to be a distinct spe- 

 cies, but in reality it is only produced by art. 



All the medullary rays are composed of cellular tissue, 

 elongated horizontally, and moderately compact. It is 

 evident that they establish a direct communication from 

 the centre to the clrcum_ference, or from the circumference 

 to the centre; but in no case are coloured juices ever 

 seen passing through them. 



The continuity of the medullary rays from their origin 

 to the circumference is very well observed when we ex- 

 amine wood which is rather soft; such as, for example, 

 the Mistletoe, or certain fleshy plants. One is sometimes 

 tempted to believe that they are continued even into the 

 bark ; and this opinion has been maintained by several 

 naturalists. Some have spoken of the medullary rays of 

 the central system as being continuous with those of the 

 cortical ; others as being distinct. Mirbel and Du- 

 trochet have given great weight to this last opinion ; and 

 if it be admitted that the two systems are essentially dis- 

 tinct at their origin, we are obliged to conclude that in 

 the cases where the rays seem to pass into one another, 

 it is only the contiguity, and not the continuity of their 

 extremities. 



Art. II. — Of the Cortical Body or System. 



§ 1. — General Considerations. 



The Cortical System of Exogens is organized upon a 

 similar plan to the Central System, but inversely as to 

 the time of the development of the layers. It is formed 



