320 THE PRIMARY STEM 



turgidity of its living cells. This is shown by the 

 drooping of many stems, or of their young upper portions, 

 when they are wilted. This source of rigidity is, however, 

 supplemented in many plants by the coUenchyma of 

 the outer cortex, which is differentiated early, that is 

 fairly close behind the apical growing point, and stiffens 

 the surface layers of the stem. This surface stifEening 

 resists slight bending strains to which the upper part oi 

 the shoot is subjected by the wind. Soon the lignifica- 

 tion of the primary xylem elements increases the support 

 afforded by the collenchyma. But the most important 

 mechanical support of most primary herbaceous stems 

 is contributed by the fibres of the pericycle. These are 

 thickened later, i.e. further from the growing point, 

 than the collenchyma, after growth in length has ceased. 

 Owing to the relative narroAvness of the cortex compared 

 with the width of the stem, the fibres of the pericycle, 

 which often have very thick walls, form a rigid continuous 

 or interrupted cyUnder not far from the surface, and 

 enable the stem to support the weight of leaves and 

 branches and to resist the bending strains imposed by 

 the wind. 



. Apical Meristem. — A longitudinal section through the 

 tip of the shoot shows the stem tip composed of meriste- 

 matic (embryonic) cells, and bearing on its sides the 

 first beginnings of the leaves, which arise as projections 

 on the surface caused by locally increased cell division. 

 As the young leaves increase in size they usually grow 

 faster on the lower than on the upper surface, with the 

 result that they curve over the growing tip of the stem 

 with its younger and as yet smaller leaves (Fig. 53), 

 thus serving to protect these and the meristem itself, 

 which are not as yet covered by a well-developed cuticle 

 from drying up and other injury. This curving of the 



