STRUCTURE Or THE STEM 



95 



Fig. 67. — Collenchymatoua 

 andOtherTissuefromStem for instance, 

 of Balsam (Impatiens). 



e, epidermis ; c, coUenchyma; 

 i, intercellTilar spaces be- 

 tween large parencliyma- 

 cells; 



chymatous tissue. In some (particu- 

 larly in flesliy)_stems the stiffness is, 

 however, largely due to coUenchyma, a 

 kind of parenchyma in which the cells 

 are thickened or reinforced at their 

 angles, as shown in Fig. 67. 



107. Early History of Stem-Struc- 

 ture. — In the very young parts of 

 stems, such, 



as the grow- 

 ing point 

 between the 

 two rudi- 

 mentary leaves of a bean-plumule, 

 the cells are all of thin-walled 

 formative tissue and look a good 

 deal alike. This condition of 

 things is quickly succeeded by 

 one in which there is a cylinder 

 (appearing in cross-sections of the 

 stem as a ring) of actively growing 

 tissue X (Fig. 68,^), lying between 

 the cortex r and the pith m. Soon 

 the cylinder x develops into a 

 series of separate fibro-vascular 

 bundles arranged as shown in 

 Fig. 68, B, and these again in a 

 short time unite, as shown at C. 

 A comparison of this last portion 

 of the figure with that of the 



Fig. 68. — Transverse Section 

 through the Hypocotyl of the 

 Castor-Oil Plant at Various 



Af after the root has just ap- 

 peared outside the testa of the 

 seed ; B^ after the hypocotyl is 

 nearly an inch long; C, at the 

 end of germination ; ■#', cortex 

 (undeveloped bark); m, pith; 

 st, medullary rays; fv, fibro- 

 vascular bundles ; c&, layer of 

 tissue which is to develop into 

 cambium. (Considerably mag- 

 nified.) 



