Lecture XXV. 219 



reduced. They fit together closely and their tapering ends overlap 

 one another for a considerable distance. The total cross section 

 of the ensheathing tube enclosing the conducting tract is small. 

 Its greatest diameter does not exceed 0*5 mm. Hence, by its 

 own stiffness it contributes little to the rigidity of the stem. Its 

 stiffness is further lessened by the fact that it is always extremely 

 reduced in thickness and often even discontinuous along two lines, 

 one at each side of the conducting tract. The fact is the sheath 

 is not constructed to oppose transverse stresses, under which it is 

 quite flexible, but to take up and resist longitudinal stresses. It 

 is a tension member, while the cells of the fundamental tissue, dis- 

 tended by osmotic pressure, form the compression member of the 

 rigid system. Thus in the rigidity which enables the stem to 

 stand erect the bundle-sheaths play a part analogous to the 

 muscles of the htiman body, while the fundamental tissue has the 

 role of the skeleton. The case is parallel to that of the leaves, 

 which we have already noticed, and may be similarly demonstrated. 

 Destroy the resistance of the fundamental tissue to compression, 

 and without altering the mechanical properties of the sheaths, the 

 stem collapses. As we have seen, this may be accomplished by 

 rendering the protoplasm of the cells permeable by poisonous 

 vapours or by heat. A protective function of the sheath is exer- 

 cised on behalf of the strands of the conducting tract and especially 

 in favour of the bast. The outer section of the sheath forms an 

 arched tunnel in which the bast is protected from the pressure of 

 the cells of the cortex, which at times, owing to the bending of the 

 stem, may be considerable. The woody tubes are better adapted 

 by their own structure to resist this pressure, and hence the inner 

 half of the bundle-sheath is less developed. 



As we have seen, the general arrangement of the tissues in the 

 mature stem of the Buttercup resembles that of the young stem of 

 Pinus. Changes take place subsequently which make the structure 

 of the older stem of the Pine more complicated. Hence, we might 

 expect the tissues of the strands of the conducting tracts would be 

 less complicated than those of Pinus. Microscopic observation 

 does not confirm this surmise. The constituents of the bast and 

 wood tissues of Ranunculus are more various and more differenti- 

 ated from one another than those of Pinus. 



The bast is divisible into the protophloem and primary bast. The 

 former lies next the outer sheath. It is composed of thin-walled, 

 elongated, tubular cells, all similar to one another, and containing 

 a watery sol of proteins enclosed by the thin film of protoplasm 

 lining their walls. The protophloem is a very thin strand containing 



