ENDOGENOUS OR MONOCOTYLEDONOUS STEM. 53 



the bundles are large, but as they descend they usually become more 

 and more attenuated. In some instances, however, as in Ceroxylon 

 andicola, they increase at different parts of their course, probably by 

 interstitial growth, and give rise to irregular swellings of the stem. 

 This distension takes place occasionally at the base of the stem, as in 

 Euterpe montana. 



96. There are many herbaceous plants in this country, as Lilies, 

 Grasses, &c., having endogenous stems, in which the course of the 

 vascular bundles may occasionally be traced, but there are no British 

 endogenous plants with permanent aerial woody stems. All the British 

 trees are exogenous. Illustrations of endogenous stems must therefore 

 be taken from trees of foreign countries. Palms furnish the best exam- 

 ples. In them the stem forms a cylinder of nearly uniform diameter 

 throughout. The leaves are produced from a single terminal and cen- 

 tral bud, called a Phyllophor, or Pkyllogen, (tpv^ov, a leaf, and q>i%u, I 

 bear, or */tvvx.u, I produce). Connected with the leaves are the vascu- 

 lar bundles, and the bases of the leaves remain attached to the outer 

 part of the stem, surrounded by the mattulla or reticulum (^[ 57). 

 While the leaves produced by one bud decay, another bud is developed 

 in the centre in a similar manner. As the definite vascular bundles 

 are produced, the stem acquires increased thickness, but it is arrested 

 in its transverse diameter at a certain epoch. The bundles, although 

 developed progressively, do not multiply indefinitely ; and thus a 

 Palm-stem seldom becomes of great diameter. 



97. In consequence of this mode of formation, the outer part of a 

 Palm-stem is the hardest and densest, and after acquiring a certain 

 degree of solidity, it resists all further distension, and frequently be- 

 comes so hard as to resist the blow of a hatchet It has been already 

 stated, that in the exogenous stem, provision is made for unlimited 

 extension laterally, by the development of indefinite bundles of woody 

 fibres and vessels, and the formation of a separable bark which can be 

 thrown off; but in the endogenous stem there is no such provision. 

 Hence, when the first-formed part of the stem has increased to a cer- 

 tain amount, its progress is stopped by the hard mdistensible outer 

 fibrous covering ; and the same thing takes place with the other parts 

 in succession, till at length all have acquired a comparatively uniform 

 size, as is seen in the coco-nut palm (fig. 115, 1). In consequence of 

 the small lateral increase of Palm-stems, a woody twining plant does 

 less injury to them than to trees of exogenous growth. 



98. The growth of endogenous stems may be said to resemble the 

 upward growth of Exogens by terminal buds only, for there is no cam- 

 bium layer, and no peripherical increase. Hence, in Palms, the ter- 

 minal shoot is developed, but there are no annual rings. The harden- 

 ing of the stem depends, in all probability, partly on internal changes 

 in the woody fibres, similar to what takes place in the heart- wood of 



