100 MECHANICAL SYSTEM 



vascular bundles, each accompanied by a strong fibrous sheath, while 

 the cortex is also traversed by numerous longitudinal fibrous strands 

 of various sizes. The mechanical system is thus distributed very 

 evenly over the entire cross-section, an arrangement which can only be 

 justified on the score that both tension and compression have to be 

 provided for. 



Straight spines and thorns must also be capable of resisting longi- 

 tudinal compression. According to Schwendener, this condition is 

 satisfied, at any rate among Monocotyledons (e.g. Agave americana, 

 Cha/maerops hum His, Zivistona sinensis). 



D. ARRANGEMENTS FOR RESISTING SHEARING STRESSES. 



When the action of a mechanical force upon a body is such that 

 the constituent particles of the latter tend to become displaced with 

 reference to one another, the force is said to be a shearing one ; the 

 property inherent in a body which enables it to resist such shearing 

 action, or which in other words prevents the component particles 

 from sliding over one another, may be termed shear-resistance. Every 

 flexure of a cylindrical or flat organ is accompanied by shearing stresses ; 

 in the case of a simple flexure, these stresses are comparatively 

 insignificant and do not necessitate the provision of any special shear- 

 resisting arrangements. When such adaptations are nevertheless 

 found to occur, especially in flattened organs, the reason must be 

 sought in the increased shearing action consequent upon violent 

 movements of the surrounding air or water. A leaf fluttering 

 in the wind is exposed to shearing stresses acting at right angles 

 to the surface and tending to cause laceration. In order to cope 

 satisfactorily with this danger, the girders that serve to ensure 

 inflexibility must be firmly anchored together by the greatest possible 

 number of cross-ties. This office is performed by the vascular 

 anastomoses to which reference has already been made. These 

 anastomoses form a dense reticulum in the leaves of Monocotyledons 

 and Dicotyledons; they are seen with the greatest clearness in "nature- 

 printed " figures of leaves. In these anastomoses the vascular tissues 

 are often accompanied by bast-fibres: in the leaf oiMaranta arundinacea 

 indeed the majority of the anastomoses consist of stereides alone. In 

 this connection mention may also be made of the " false veins " 

 described long ago by Mettenius which traverse the delicate fronds 

 of certain species of Trichomanes : these are also entirely composed of 

 mechanical elements. 



It is, of course, the leaf-margin which is particularly exposed to the 

 risk of tearing, and which therefore most frequently requires special 

 mechanical protection. 104 The simplest, and at the same time the most 



