THE STELE 113 



and "lamellar" collenchyma. Notice that intercellular air spaces 

 are extremely small and difficult to distinguish. 



(//) An inner region composed of typical "isodiamet- 

 ric" parenchyma cells with active protoplasts and cells containing 

 druses of calcium oxalate. The latter type of cells tend to occur 

 in groups (refer to the radial section of the stem) and their 

 specialized character has earned for them the term of "crystal 

 sacs." It is important to notice the crushed appearance of many 

 of the parenchyma cells in the cortex ; this condition has been 

 caused by the pressure of the secondary phloem which is con- 

 stantly pushed toward the periphery of the stem. 



(d) The stele. Tilia possesses an ectaphloic sij^honostele but 

 the arrangement of the tissues of the secondary phloem show^s a 

 number of features distinctive of this genus. The phloem occurs 

 just Avithin the innermost layer of the cortex and consists of a 

 cylinder made up of wedges of tissue showing a characteristic 

 "banded" appearance which alternate with triangular sectors 

 which are "homogeneous" in structure; the wedges showing the 

 band-like structure are broadest next to the cambium while just 

 the reverse relationship obtains in the case of the homogeneous 

 sectors. 



Examine in detail, first of all, the structure of one of the large 

 '^lianded" sectors. You will observe that the characteristic 

 "banded" appearance of such a sector is due to the alternation 

 of tangential layers of extremely thick-walled fibers with layers 

 of thinner walled cells. These thick-walled phloem fibers, which 

 provide mechanical support and flexibility to the stem, have long 

 been known as "bast fibers" and their prominent development 

 in the genus Tilia is responsible for the old English name of this 

 tree, i.e., * ' bast-wood ' ' which in modern language has been changed 

 to "basswood. " Under high power, the phloem or "bast fibers" 

 appear irregularly polyhedral, are closely joined without evident 

 intercellular air spaces, and possess extremely small lumina ; under 

 especially favorable conditions of magnification and illumination 

 you may be able to see the infrequent canal-like pits between ad- 

 jacent cells. 



The layers of thinner-walled cells alternating with the bands 

 of "bast fibers" are composed of the conducting and storage cells 



