Structural Botany : The Woody 'Stem VIII. 



The massive timber-tree which may commonly attain a height of 100-200 ft., 

 and live for hundreds of years, yet may take 20-30 years to attain reproductive 

 maturity, is in no sense as efficient an organism as the rapidly growing herbaceous 

 annual with quick returns. Indigenous trees are characteristically deciduous, and 

 work in terms of a short season (May-October), enduring the vicissitudes of the 

 winter. The amount of seasonal increment is relatively small; the tissues are 

 densely sclerosed, giving rigidity to the axis, and the cell-units remain typically small 

 and closely compacted. 



The vascular cylinder from the beginning is preponderant in the stem ; the 

 xylem constituting a close cylinder around the pith ; individual V. B. are not noticed. 

 Growth continues by a circular cambium-ring, external to which narrow zones of 

 phloem and cortex are popularly but incorrectly considered as ' bark ', because they 

 can be peeled off at the cambium. 



Special features require detailed notice : 



I. Cambium cells have the shape of greatly elongated rectangular prisms, with 

 definite orientation, and may divide by transverse, radial, and tangential walls 

 respectively ; i. e. in 3 dimensions. 



(a) Transverse walls, beyond the zone of primary extension of the axis, merely 

 produce septation into shorter segments in longitudinal series, as in medullary-ray 

 formation, and production of xylem parenchyma. 



(/?) Radial walls increase the number of initial units, giving more radial rows of the 

 same size to fill the circle as the stem increases in circumference. 



(y) Tangential walls give new tissue-units, as phloem on the external side (centri- 

 petal), and xylem on the inner side (centrifugal). Each division involves mitosis, and the 

 formation of a new cell-wall : in this last case in the position presenting greatest 

 difficulty. Initial cells (i. c.) in each radial row now control the mechanism, with 

 some sort of power of choice as to which segment shall continue as initial. 



II. Annual Rings. Increase by cambium may continue indefinitely, and one 

 cambium may persist throughout the life of the tree as a permanent meristem of 

 embryonic activity, but restricted to special tissue-production : a cambium does not 

 originate any other part of the plant (as a branch or root). Where annual periods 

 are marked by climatic changes, tree-types respond by seasonal periodicity. T.he 

 cambium may be dormant, or ' resting ', at certain periods of the year, and start again 

 with renewed activity (as in Spring of the N. Temp.). Differences in the tissues 

 formed at different seasons may give the effect of zones in the wood; i.e. anew 

 growth each year as an annual ring. Spring wood is commonly indicated by wider 

 vessels ; summer wood by smaller vessels and tracheides, abundance of fibres and 

 increase in deposit of polysaccharide. Decreasing size to a minimum in late summer, 

 followed by sudden growth of larger units in the spring, with special function, 

 emphasizes the ring-effect. Such rings vary in width from 1-20 mm., as annual 

 increments, and afford a clue to the age of the tree, giving the ' grain ' of timber as 

 more or less concentric circles ; though again naturally wanting in trees growing with 

 little change of season. 



III. Medullary Rays : The spaces between the primary bundles of a young 

 stem are conveniently known as primary rays. These tend to be obliterated by 

 interfascicular extensions. Secondary rays are more important, as formed at and by 

 the cambium for a special purpose. Any initial cell may be set apart to produce 

 a ray. These rays are distributed at fairly constant intervals (varying in different 

 types) ; and as the circle of cambium increases, new rays are initiated to maintain equal 

 distribution, midway between the older ones. Being initiated in the cambium, the rays 

 differentiate both ways in ^ and < for ever after, following radial lines at right-angles 

 to the curve of the cambium ' circle ' ; and so with the concentric rings constituting an 

 orthogonal geometrical construction, as seen in cross-section, centric or eccentric, 

 according to the growth of the stem. 



Typically of living parenchymatous units, radially elongated, more or less 

 sclerosed in the xylem, and storing starch, tannin, or crystals, the primary function is 

 to act as living units controlling the mechanism of radial and transverse conduction to 

 other living cells of the vascular cylinder. Commonly only i cell wide, and several 



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