96 



ENDOGENS. 



direction, but form arcs whose extremities pass upwards and downwards, 

 losing their extremities in the leaf on the one hand, and on the other in the 

 roots, or in the cellular integument on the outside of the first circle of cords ; 

 at the same time the second leaf pushes the first leaf a little from the centre 

 towards the circumference of the cone of growth. In this manner leaf after 

 leaf is developed, the horizontal cellular system enlarging all the time, and 

 every successive leaf, as it forms at the growing point, emitting more woody 

 bundles curving downwards and outwards, and consequently intersecting the 

 older arcs at some place or other ; the result of which is that the first formed 

 leaf will have the upper end of the arcs which belong to it longest and much 

 stretched outwardly, while the youngest will have the arcs the straightest ; 

 and the appearance produced in the stem will be that of a confused entangle- 

 ment of woody bundles in the midst of a quantity of cellular tissue. As the 

 stem extends its cellular tissue longitudinally while this is going on, the 

 woody arcs are consequently in proportion long, and in fact usually appear 

 to the eye as if almost parallel, excepting here and there, where two arcs 

 intersect each other. As in all cases the greater number of arcs curve out- 

 wards as they descend, and eventually break up their ends into a multitude 

 of fine divisions next the circumference where they assist in forming a cor- 

 tical integument, it will follow that the greater pai-t of the woody matter of 

 the stem will be collected near the circumference, while the centre, which 

 is comparatively open, will consist chiefly of cellular tissue ; and when, as 

 in many palms, the stem has a limited circumference, beyond which it is 

 its specific nature not to distend, the density of the circumference must, it 

 is obvious, be proportionally augmented. It is however a mistake to suppose 

 that the great hardness of the circumference of old palm wood is owing 

 merely to the presence of augmenting matter upon a fixed circumference ; 

 this will accoimt but little for the phenomenon. We find that the woody 

 bundles next the circumference are larger and harder than they origi- 

 nally were, and consequently we must suppose that they have the power of 



increasing their own 

 diameter subsequent 

 to their first forma- 

 tion, and that they 

 also act as reser- 

 voirs of secretions of 

 a hard and solid na- 

 ture, after the man- 

 ner of the heartwood 

 of exogens. 



When the growth 

 of the stem of an endogen goes on in this regu- 

 lar manner, with no power of extending horizon- 

 tally beyond a specifically limited diameter, a 

 trunk is formed, the sections of which present 

 the appearances shown in the accompanying 

 cut. There is a number of curved spots crowded together in a confused M-ay, 

 most thick and numerous at the circumference, comparatively small and 

 thinly placed at the centre ; and the only regular structure that is observable 

 with the naked eye is that the curves always present their convexity to the 



Rg. LXIX. 



Pig LXX. 



Fig. LXIX.— Horizontal section of a Palm stem. 

 -Lonyitiuliua) section of a Palm tree. 



