192 VEGETABLE ORGANOGRAPHY. 



We have seen above that when Exog;enous trees are 

 surrounded by cord or wire, they have a contraction in 

 that part on account of their increase in diameter. It 

 is evident, that since the diameter of Pahns only increases 

 in their young state, they are protected from this acci- 

 dent during the rest of their hves : this explains a phe- 

 nomenon which I have witnessed, of a Palm which has 

 become surrounded at a great height by a Bauhinia, the 

 branches of which, in uniting together, enclosed it in an 

 irregular interrupted sheath, without having altered the 

 cylindrical form of the trunk. 



All this assemblage of rectilinear fibres, of which I 

 have spoken, is surrounded by a zone of cellular tissue, 

 which may be compared to the cellular envelope of 

 Exogens; but it however presents remarkable diffe- 

 rences: — I. Nothing representing the cortical layers is 

 found beneath this envelope ; 2. Under it, at least in the 

 Palms with simple stems, no woody layer exists : and it 

 is very doubtful whether it be formed even in the small 

 number of Palms which are branched. 3. This envelope 

 not being distended beyond a certain limit by the increase 

 of the trunk, preserves for a much longer time its thick- 

 ness and form ; it is usually thin, and cannot at any period, 

 in the Palms, be separated from the trunk, and no trace of 

 medullary rays is ever found in it more than in the trunk. 



If the growth of the stems of Palms be compared with 

 that of Exogens, we see — 1. That the increase in 

 length is produced in the two cases by the formation and 

 the development of a terminal bud, which lengthens the 

 already existing trunk ; — 2. That the increase in dia- 

 meter can take place until a fixed age in each species, 

 either by the dilation of each bundle of fibres by means 

 of fibrous or cellular tissue, which is interposed ; or by 

 the development of new bundles tov/ards the centre of 

 the tree. 



