150 



Elementary Biology. 



intercellular communication occurs in a great number of 

 tissues besides the sieve-tubes. Among the vessels of the 

 wood, as already stated, a great variety of types exist, known 



as sftiral. annular. 



FIG. 68. SIEVE-TUBES FROM PHLOEM OF Cucurbita 



(Sachs.) reticulated, scalari- 



forrn, and other ves- 

 sels. 



It is important to 

 notice, in regard to 

 the fibro - vascular 

 strand of a monoco- 

 tyledon, that there is 

 no growing tissue 

 present it has all 

 become permanent 

 tissue. The strand 

 is incapable, there- 

 fore, of growing larger 

 when once formed ; 

 the strand is there- 

 fore said to be closed. 

 We shall find this an 

 important point of 

 distinction between 

 the monocotyledo- 

 nous and the dico- 

 tyledonous type of 

 stem, in which latter 

 growing tissue or 

 cambium is present, 



q, transverse section of a sieve ; sL sieve on side 

 wall ; x, pits on face and /, on side view ; ps, 

 protoplasm contracted from the cell-wall ; z, in- 

 termediate prosenchyma. 



and where the fibro- 

 vascular strands are 

 therefore known as 



open strands, i.e. capable of further growth. 



Hoot. Passing next to the root, we find that the struc- 



ture differs considerably from that of the stem, not in the 



