668 



A TEXTBOOK OF THEORETICAL BOTANY 



peculiar in having pits on its tangential walls as well as on the radial walls, 

 thus ensuring continuity between successive rings. 



The secondary phloem is made up largely of radial rows of sieve tubes 

 with many lateral sieve areas, on the radial walls, which become covered 

 with callus after their first year. Some scattered parenchyma cells are also 

 present. 



So far we have only considered the structure of the stele ; we must now 

 turn to the cortex. In the ver^^ young stem the cortex is made up of paren- 



FiG. 667. — Pinus sylvestris. Three-dimensional model of a 

 block of wood showing one annual ring with wide 

 tracheids in the spring wood and narrow tracheids in 

 the autumn wood. The medullary rays are also seen 

 in transverse and longitudinal views. {From a model 

 in the National Museum of Wales.) 



chymatous cells, and since the endodermis and pericycle, if present, are 

 composed of the same type of cells it is impossible to recognize them as 

 separate layers. As growth continues, a layer of cortical cells near the outside 

 becomes active and forms a meristematic layer termed the cork cambium 

 or phellogen. This layer cuts off additional cortical cells towards the inside 

 and periderm or cork towards the outside. This cork layer, which con- 

 stitutes the bark, is impervious to water and serves to protect the stem from 

 excessive evaporation. It also provides a protection to the more delicate 

 tissues within. 



All these structures are also found in the stem of the x\ngiosperms, and we 

 shall consider them again in more detail when we come to study the anatomy 

 of that group. In Pinus and in many other Conifers belonging to the Abietineae 

 large resin canals are present in the cortex opposite each primary vascular 



