THE STEM OF VASCULAR PLANTS. 189 



and surrounded by a loose and feculent cellular tissue. 

 Each fibre is a bundle of tracheae and striped and dot- 

 ted vessels, intermixed with elongated, and surrounded 

 by round cellular tissue. The difference in texture 

 between the circumference and centre of the trunk is 

 always perceptible — sometimes very remarkably so; 

 there are some Palms, the outer part of which is so hard 

 that the axe can make no impression on them, whilst 

 the centre is a loose and spongy tissue, which is readily 

 affected by moisture. The circumference of Palms repre- 

 sents, as to consistence and age, the wood of our trees, 

 whilst the centre is a kind of alburnum. But these 

 two organs are placed inversely to that which we are in 

 the habit of seeing in Exogens : it is from this central 

 alburnum that the leaves and flowers spring ; it is, in a 

 word, always from the centre that the development of 

 all parts of Palms commences. The young leaves of the 

 annual shoots of Exogens also spring fi-om wdthin the 

 older ones, or from the interior of the buds ; but if in 

 this respect the two great classes resemble each other, as 

 Dutrochet has rem^arked, they do not differ less in this, 

 that all the rest of the development of the trunk of 

 Exogens is made by the addition of new woody layers 

 outside the first ; whilst in Endogens, the increase takes 

 place by the interposition of new fibres, principally 

 towards the centre of the trunk. 



At the first development of the plant, the first row of 

 leaves is connected with the neck by a layer of fibres ; 

 in the second year, there springs from the interior of 

 this first row a second row of leaves, which have also a 

 layer of fibres placed within the preceding, which, by 

 their development, help to distend the first layer. It is 

 the same with all the following layers, until the period 

 when the external one, having acquired by age the hard- 

 ness of perfect wood, can no longer be distended by the 



