34 Dr. W. C. Williamson on 



All the yet more external zones of tissue constitute the 

 Cortex, supporting the leaves on its outer surface. In its 

 youngest state this tissue consists almost wholly either of 

 rounded cells (parenchyma), or of vertically elongated ones 

 with pointed ends (prosenchyma). At a more advanced 

 stage of growth, the commencement of which varies in 

 different types, a thin meristemic zone makes its appear- 

 ance in the outermost parenchyma of the Cortex ; an 

 irregular ring of its rounded cells, seen in transverse sections, 

 undergo divisions : the more internal of these new cells are 

 developed into prosenchymatous ones, thus causing the 

 formation of a layer of '■'periderm" which continues to grow 

 by similarly produced additions to its exterior so long as 

 the plant continues to live. It is this layer that constitutes 

 the great bulk of the gigantic stems, some of which have a 

 circumference of ten or twelve feet, and at the bases of 

 which this periderm attains its greatest thickness. The 

 outermost of the cells produced by the above meristemic 

 action again undergo a succession of similar metamor- 

 phosis, so that whilst the periderm enlarges, there always 

 remains a thin layer of parenchyma between the periphery 

 of that periderm and the bases of the leaves. These 

 leaves vary much in the different types, being sometimes 

 elongated and narrow, and in other forms they are very 

 short and broad ; the bases of these leaves, which are 

 small and nearly square in their young state, becoming 

 very much enlarged and protuberant as growth advances. 

 Eventually the true leaf falls off, leaving its base, known as 

 the leaf cushion, behind it, but on the point from which the 

 leaf was detached we find it has left the leaf-scar> a 

 quadrangular area of varied shapes and sizes upon which 

 we discover three well-marked points. These leaf bases 

 are more often than otherwise arranged upon the aged 

 stem in diagonal lines, though in others these lines are 

 conspicuously vertical ; but even in these latter the diagonal 



