PHYSIOLOGICAL CLASSIFICATION OF TISSUES. 193 



531. The distribution of material in the skeleton of a ligneous 

 dicotyledonous plant is somewhat different from that in a mono- 

 cotyledon. 1 More of the mechanical work falls on the proper 

 wood, but even here in some cases the bast serves an important 

 purpose. 



532. The data for calculating the strength of the wood}' stem 

 and branches of a dicotyledonous plant are to be found in vari- 

 ous works on mechanical engineering ; but it is to be borne in 

 mind that the figures given for timber are usually based on ex- 

 periments with dry heart-wood. 



533. The trunk is to be regarded as a column bearing the 

 weight of the whole crown of branches, each of these being a 

 tapering beam supported at one extremity. The crushing-weight 

 the crown exerts upon this column is far within the limits of 

 safety, even when the liability of the trunk to be much bent and 

 twisted by high winds is taken into account. The branches at 

 their point of union with the trunk form different angles in 

 different plants,' 2 and this angle must be taken into consideration 



1 DISTRIBUTION OF MECHANICAL ELEMENTS IN DICOTYLEDONS. 



1 . With bast in the bark. 



First group. Axial organs when young have an unbroken ring of bast ; 



in much older stems this is interrupted or cast off. Aristolochia. 

 Second group. Axial organs with a layer of bast-bundles which is 



thrown off later. The bast-bundles form the first mechanical system, 



which is soon replaced by the ring of wood. Nerium Oleander. 

 Third group. With simple ring of bast-bundles in first year, later with 



isolated bast-fibres. ^Esculus Hippocastanum. 

 Fourth group. With strong bast, even when far advanced. Tilia. 

 Fifth group. With subepidermal bast-nerves. Eusselia. 



2. With transition to an intra-cambium ring of libriform cells. 



Sixth group. The cambium of the bundles lies partly outside, partly 



inside the mechanical ring, or is imbedded therein. Gaillardia. 

 Seventh group. Isolated vascular bundles. Silphium perfoliatum. 



3. Intra-cambium libriform ring without medullary rays. 



Eighth group. Without bast on the outer side of the cambium or cam- 



biform layer. Impatiens Nolitangere. 

 Ninth group. With larger or smaller amounts of bast on the outer side 



of the cambriform. Urtica dioica. 

 Tenth group. In the libriform elements all shades of transitions to 



ducts. Mirabilis Jalapa. 



4. Intra-cambium libriform ring with parenchyma rays. 

 Eleventh group. Rays formed of elongated cells. * Vinca major. 

 Twelfth group. Typical dicotyledons with medullary rays. 



2 McCosh has given the angles in a large number of plants, a few of which 

 are here cited : Ash, 60; horse-chestnut, 50 -55: alder, 50; elm, 50; oak, 

 large branches, 50, small branches, 65- 70 : be^ch, 4o: linden, 40. He calls 

 attention to the fact that in these and many other cases the angle at which the 



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