STRUCTURE OF THE STEM 



95 



OOL 



FIG. 67o Collenchymatous 



and Other Tissue from Stem for instance, 



of Balsam (Impatient^. ag the grow . 



e, epidermis ;c, coll enchyma; . 



i, intercellular spaces be- in g p 1 Ht 



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 FlG 

 tissue x (Fig. 68, A), 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 0. 

 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 



Transverse Section 

 through the Hypocotyl of the 

 Castor-Oil Plant at Various 

 Stages. 



, 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, fibro- 

 vascular bundles ; cb, layer of 

 tissue which is to develop into 

 cambium. (Considerably mag- 

 nified.) 



