696 SECONDARY GROWTH IN THICKNESS 



led Schenck to conclude that this peculiar feature is not an adaptation 

 to the climbing habit, but is mainly a question of variety of design. 

 The fact remains, that inter- xylary leptome is of particularly frequent 

 occurrence in climbers belonging to the most diverse families. 



The remaining histological peculiarities of liane-stems may be 

 disposed of in a few words. Westermaier and Ambronn long ago drew 

 attention to various features which represent anatomical responses 

 to increased demands in the matter of translocation. The remarkable 

 width of the vessels and sieve-tubes in the stems of climbing plants has 

 been discussed in detail on a previous occasion (pp. 324, 334). "Whether 

 Westermaier and Ambronn are right in regarding the great develop- 

 ment of xylem-parenchyma, and the often unusual vertical and tangential 

 extent of the medullary rays, as further devices for increasing the rate 

 of translocation is another question ; in Schenck's opinion the last- 

 mentioned anatomical feature serves to facilitate the bending and 

 twisting of the stem. 3 '" 



It has already been explained that, in their later stages of develop- 

 ment, liane-stems often differ from ordinary upright woody stems, even 

 as regards their external form. By far the most remarkable illustra- 

 tion of this point is provided by the greatly flattened ribbon-shaped 

 stems of certain Leguminosae (RJu/nchosia, Dalbergia, and Machaerium 

 among the Papilionaceae ; Bauhinia among the Caesalpinieae), which 

 owe their characteristic shape to an unequal distribution of cambial 

 activity, and in some cases also to the appearance of successive 

 cambial layers. Such ribbon-shaped stems are sometimes thrown into 

 folds in a most extraordinary manner ; often the two edges take no 

 part in the corrugation. The growth -processes responsible for this 

 peculiar development have been studied and discussed by Cruger, De 

 Bary, Von Hohnel, Warburg, and Schenck, but are still imperfectly 

 understood. The ecological significance of the undulations is also not 

 quite clear. Schenck supposes that the numerous abrupt curvatures 

 assist the stem to gain a fresh hold, if it slips down through its tendrils 

 letting go or the support giving way. With regard to those ribbon- 

 shaped stems which are not corrugated, Schenck points out that a 

 flattened organ will lodge more readily upon the supporting branches 

 than a round one. 



///. SECONDARY GROWTH IN THICKNESS OE 

 FLESHY ROOTS 



The roots of a great many biennial and perennial Dicotyledons act 

 as organs of storage [and hibernation] after the aerial structures have 

 withered, and in accordance with this function assume a " fleshy ' 



