TISSUES OB' ANGlOSPERMtl. 



449 



treme cases more than a thousand,* the bark rarely shows 

 more than a few distinct layers, and its thickness is generally 

 very much less than that of the former. 



From what has been said it is seen that a dicotyledonous stem several 

 years old is composed of a series of larger and larger continuous woody 

 shells (Fig. 330, 1, 3, 3, 4, 5) surrounded by a corresponding series of 

 bark shells, which are smaller aud smaller (Fig. 330, 5', 4' 3', 2', l^. 



548.— The Medullary Rays, In the young dicotyledonous 

 stems there are thick masses of parenchyma, which connect 

 the cortical with the medullary (pith) portion of the funda- 

 mental system of tissues (Fig. 323). However, as the fibro- 

 vascular bundles increase, 

 these masses become thin- 

 ner, until they are mere 

 plates, often not more than 

 one or two, or at most a 

 few cells in tjiickness (Figs. 

 326-7-8). From their ap- 

 pearance and position they 

 have long borne the name 

 of Medullary Eays. In 

 the young stem their cells 

 may be parenchymatous, 

 but in older ones they are 

 frequently sclerenchyma- 

 tous. Viewed in a radial 

 section of the stem, they are generally seen to be elongated 

 in the direction of the radius, having the outlines of right- 

 angled quadrilaterals. In the increase of the diameter of the 

 stem there is always an increase in the length of the medul- 

 lary rays, both in their bark and wood portions ; and when 

 from their divergence a considerable space intervenes between 

 two rays, one or more new ones arise between them ; thus 

 while there may be no more than four or five rays in the 

 young plant, it may when old have hundreds of them m its 

 circumference (Fig. 329). 



What has been siiid of the tissues of the Angiosperms must suffice to 



Fig. 339. — Cross-section of the stem of an 

 oak {Quercun Rubur) thirty-seven years old. 

 m, pith ; Ig. heart-wood ; Ig', sap-wood ; rm, 

 mednllary rays ; ec, the bark. Much reduced, 

 — After Duchartre. 



* In the Lime (IMia Europmi) 1076 and 1147, and in the Oak (Quer- 

 cus Boiur) 1080 and 1500, according to De CaudolU-. 



