ENDOGENOUS OE MONOCOTYLEDONOUS STEM. 67 



periphery, while in the latter they are arrested in their transverse 

 growth at a definite epoch. The investing bark of the former permits 

 an unlimited extension of woody growth beneath it ; the fibrous cor- 

 tical layer of the latter, by maintaining an intimate union with the 

 subjacent tissue, prevents unlimited increase in diameter. Hence we 

 find that true endogenous stems do not attain the enormous diameter 

 exhibited by some exogenous trees, such as Sequoia (Wellingtonia) 

 gigantea and the Baobab, — the former of which has been measured 

 116 feet in circumference. 



The composition of the vascular bundles, in different parts of 

 their course, varies. Thus, at the upper part, tracing them from 

 the leaves towards the centre, they contain spiral vessels, pitted vessels 

 with some cellular tissue, a few laticiferous vessels, and woody fibres 

 resembling those of liber (fig. 131). As we descend, the spiral vessels 

 disappear, then the pitted vessels; and when the bundles have reached 

 the periphery, and have become incorporated with it, nothing but 

 fibrous tissue, or pleurenchyma, remains, forming a complicated ana- 

 stomosis or network. Thus, at the commencement, the bundles are 

 large, but as they descend they usually become more and more atten- 

 uated. 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 disten- 

 sion takes place occasionally at the base of the stem, as in Euterpe 

 montana. 



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

 Grasses, etc., 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 examples. In them the stem forms a cylinder of nearly uniform 

 diameter throughout. The leaves are produced from a single terminal 

 and central bud, called a PhyUophor or Phyllogen {iphWov, a leaf, and 

 (po^iTv to bear, and yivvaen, to produce). Connected with the leaves 

 are the vascular bundles, and the bases of the leaves remain attached 

 to the outer part of the stem, surrounded by the mattuUa or reticulum. 

 While the leaves produced by one bud decay, another bud is de- 

 veloped in the centre. As the definite vascular bundles are produced, 

 the stem acquires increased thickness, but it is arrested in its trans- 

 verse diameter at a certain epoch. The bundles, although developed 

 progressively, do not multiply indefinitely; and thus a Palm-stem 

 seldom becomes of great diameter. 



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 firmness it resists all further distension, and frequently be- 



