CYLINDRICAL INFLEXIBLE ORGANS 



175 



affecting the inflexibility of the stem. Those fibrous strands, on the 

 other hand, which lie on the outside of the main stereoine-cylinder, 

 probably help to prevent the cortex from splitting- or being stripped 

 off when the stem is bent. 



A very remarkable variety of the third mechanical system is the 

 " corrugated " type of stereome to use Potonie's term which is found 

 in the stems of Tree Ferns 

 (Cyathea, Alsophila), belong- 

 ing to the tropical family 

 Cyatheaceae. 97 Here the 

 mestome-bundles take the 

 form of ribbons or sheets 

 enclosed in thick stereome- 

 sheaths. Each ribbon is 

 curved in a horizontal 

 plane, being as a rule 

 roughly V-shaped in cross- 

 section, with the sharp edge 

 facing the centre of the 

 stem. Since such a curved 

 sheet opposes a much greater 

 resistance to bending strains 

 than a flat sheet of equal 

 thickness, the 



advantages 

 of this corrugated mode of 

 construction which is, of 

 course, widely employed in 

 sufficiently obvious. 



Fio. 60. 



Sub-cortical fibro-vascular strands. Peripheral portion of 

 a T.S. through the spadix of Phoenix dactylifera. Each of the 

 small peripheral fibrous strands contains an excentrically 

 placed mestome-bundle. The sub-cortical mestome-strands 

 are provided with stout external fibrous sheaths which in 

 places unite to form irregular ribbons or sheets. Towards the 

 centre of the section the fibrous sheaths of the mestome- 

 bundles become gradually weaker. 



engineering and building practice are 



(d) Fourth system. Simple hollow cylinder or tube of stereome with 

 embedded or apposed mestome-strands. 



The fourth system is so largely employed among Monocotyledons 

 that it may be regarded as the dominant type in this group of plants. 

 It is particularly prevalent in the Liliaceae (Fig. 61a, b), Iridaceae, 

 Orchidaceae and Dioscoreaceae ; but many Dicotyledons also {e.g. the 

 Chenopodiaceae, Caryophyllaceae, Geraniaceae, Primulaceae and 

 Cucurbitaceae, and the genera Phytolacca, Aristolochia, Papaver, Geum, 

 Saxifraga, Armeria, Plantago, Lonicera), conform to this scheme, so 

 that there are obvious points of contact between the two great 

 divisions of Angiosperms, in respect of their skeletal arrangements. 



In Monocotyledons the mestome-strands may be apposed to either 

 side of the fibrous cylinder, or they may be completely embedded in 

 the latter. Among Dicotyledons a series of transitions may be observed 



