STRUCTURE OF THE STEM 



95 



FIG. 67o Collenchymatous 

 and Other Tissue from Stem IOr instance, 

 of Balsam (Inpatiens). ^ ^ gmw _ 



e, epidermis ; c, collenchyma; 

 i, intercellular spaces be- in g point 

 tween large parenchyma- between the 

 cells. 



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, J.), lying between 

 the cortex r and the pith m. Soon 

 the cylinder x develops into a 

 series - of separate nbro-vascular 

 bundles arranged as shown in 

 Fig. 68, #, 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 



chymatous tissue. In some (particu- 

 larly in fleshy) stems the stiffness is, 

 however, largely due to collenchyma, 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, 



B 



FIG. 68. Transverse Section 

 through the Hypocotyl of the 

 Castor-Oil Plant at Various 

 Stages. 



A, 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; r, cortex 

 (undeveloped bark); m, pith; 

 st, medullary rays ; fv, nbro- 

 vascular bundles; cb, layer of 

 tissue which is to develop into 

 cambium. (Considerably mag- 

 nified.) 



