TiSSUES OF AN&IOSPERMS. 



449 



treme eases more than a thousand,* the bark Mrely 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 and smaller (Fig. 330, 5', 4' 3', 2', I'). 



548.— The Medullary Bays. 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. 333). However, as the fibro- 

 Tascular 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 thickness (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 in its 

 circumference (Fig. 339). 



What has been said of the tissues of the Angiosperms must sufiSce to 



Pig. 329— Cross-section of the stem of aa 

 oak (Quercut Eubur) thirty-seven years old. 

 m, pith ; Ig. heart-wood ; Iqi^. eap-wood ; rm, 

 medullary rays ; ec, the barfe. Much reduced, 

 — After Duchartre. 



* In the Lime {Tilia Europwa) 1076 and 1147, and in the Oak (Quer- 

 cus Robur) 1080 and 1500, according to De CandoUo. 



