< YLINDRICAL INFLEXIBLE ORGANS 



171 



the petioles of some Umbellifkrab as regards collencbyma. In all cases 

 the mechanical strands are disposed cither in a single zig-zag series orin 

 two concentric circles. Theadjoin- 

 ing diagram (Fig. .~>6 b) depicts 

 the arrangement of the fibrous 

 strands in the petiole of Colocasia 

 antiquorum : here the strands 

 vary in size and are regularly 

 associated with mestome-bundles. 

 The four-sided stem which is 

 characteristic of the Labiatae 

 (Fig. 56 a) is strengthened by 

 two diagonally placed collenchy- 

 matous girders; this is the 

 simplest possible type of me- 

 chanical construction in the 

 case of a cylindrical inflexible 

 organ. 



In this connection mention 

 may also be made of the 

 occurrence of a circle of fibrous 

 strands in the yearling twigs 

 of many woody Dicotyledons 



i Z' ,,, , >/,/, I , , / Subepidermal girders. A. T.S. through an internode 



[Tut//'*, aetUia, JLC&I CampeStre, | . ; the collencbymatous-strandB in the 



/ , ., , , ,,,;,,,.. A', .,',.. /)/ angles of the stem form a pair of diagoiiallv placed 



Cornus sanguined, Nenum On- Ki , -.,.. B . Partofa-rXthrongfithe^etioieof 



innl, r P,i,il,,,r,ii,i Ptn ^ nt a a antiquorum ; the fibrous strands are accom- 



uiuiu, jriniiun ' il/'i , ei<_. ) . ai a panied by mestome-bundles. <30. (After Scbwen- 



lfltpr Ql-irrp vvVipii Hip nnrimf d n er.) Both figures' diagrammatic. (In all the diagrams 



1<IU 1 Stage, wneil llie Output dealing with the mechanical system, the fibrous and col- 



Of SeCOlldarV WOOd Suffices for ^hymatous strands are distinguished by shading.) 



mechanical purposes as well as for conduction, this primary mechanical 

 system is cast oft' as a result of cork-formation. 



(/') Second system. Composite peripheral girders. 



The second form of mechanical system is especially prevalent among 

 the Cyperaceae and JUNCAGINACEAE. The six types included in this 

 system by Schwendener comprise a great variety of structural forms; 

 only three of these need be described here 



The haulm of Triehophorum germanicum is supported by a sub- 

 epidermal circle of I-girders most often live or six in number 

 which are placed at regular intervals from one another. The outer 

 flange of each girder abuts directly against the epidermis, while the 

 inner flange forms a semi-lunar sheath around the mestome-strand 

 which represents the web of the girder (Fig. 57 A). There is in addition 

 a series of more deeply seated subsidiary girders which alternate with 



