INTRODUCTION 



39 



Fig. 26. — Diagram of section of a 

 one-year-old shoot of Lime, show- 

 ing woody bundles separated by 

 broad bands of soft (parenchyma- 

 tous) tissue. Mechanical tissue 

 grey ; vascular bundles black 

 with white spots. 



woody bundles arranged round the stem are required 

 and as a matter of fact we find 

 a series of groups of woody 

 fibres ranged round the stem 

 (Fig. 26). In low -growing 

 herbs they may be widely sep- 

 arated by soft cellular tissue ; 

 and the more force the plant 

 has to resist the more they are 

 developed, and the greater pro- 

 portional space they occupy. In 

 the young shoot they are some 

 distance apart, but gradually 

 approximate (Fig. 27) until at 

 length they are only separated 

 by what are known as the 

 medullary rays. In dicoty- 

 ledonous trees a layer of wood is, as we know, laid 

 on, as it were, each year, so that the age of the tree 

 may be estimated by the rings, and the whole form 

 wedge-shaped masses of wood, separated by narrow 

 medullary rays. Being thus arranged at equal dis- 

 tances round the central neutral axis, the stem naturally 

 assumes the prevalent round form. Whatever the size 

 of the plant, whether it is a small herb, or a giant of 

 the forest, the same rule applies. 



So far as the structure of round monocotyle- 

 donous stems is concerned, the subject is dealt with 

 by Schwendener in a masterly memoir, Das mechanische 

 Princip in anatomisch Bau der Monocotylen. 



This being so, the question arises. Why do any stems 

 assume other forms ? ^ Let us take first the plants in 

 which it is quadrangular. In a species where the 

 leaves are arranged round the stalk, like the spol^es of 

 a wheel, the resistance required is equal all round (see 

 Fig. 25). On the other hand, if the leaves are "decus- 

 sate " or opposite, each pair being at right angles, this 

 above and that below, then the strain of the wind acts 



' Avebury, Me}}. Brit, Assoc. Cambridge (1905), 



