182 MECHANICAL SYSTEM 



of the large air-passages in the stems of Papyrus antiguorum, Juncns 

 glaucus, Scirpus lacustris and many other water- and marsh-plants, 

 into compartments, by means of transverse plates or diaphragms. In 

 J uncus glaucus and Scirpus lacustris these diaphragms succeed one 

 another at intervals of between 5 and 1 mm. ; as a rule they contain 

 small transverse vascular strands or mestome-anast< mioses, which link 

 up the longitudinal bundles with one another, thus representing a 

 further stiffening device. 



According to Magucsy-Dietz 10 , the septate pith which occurs in 

 many woody plants also belongs to the category of special stiffening 

 arrangements, especially when the diaphragms are largely composed of 

 sclerotic cells (Liriodcndron tulipifera). The diaphragms lose their 

 mechanical significance more and more as the twigs grow older, and 

 at the same time acquire increased importance as repositories of storage 

 material. 



4. Mechanical arrangements in connection with 

 intercalary tncristems. 



There are comparatively few Phanerogams in which cell-formation 

 and -extension are strictly confined to the apical region of the axis and 

 to the youngest internodes. Such a " direct superposition of new 

 structural elements upon an adequately strengthened foundation," as 

 Schwendener terms this mode of growth, occurs for instance among 

 Palms, in the genus Dracaena, in the Paxdanaceae, in many Liliaceae, 

 etc. Much more usually, however, the longitudinal extension of axial 

 organs is largely dependent upon intercalary growth, a condition which 

 necessitates special adaptations on the part of the mechanical system. 



In the axial organs of most Dicotyledons and many Monocotyle- 

 dons the growing region is rather extensive : it generally comprises a 

 number of internodes, while its total length may amount to anything 

 between two and fifty centimetres. As such growing regions have to 

 be inflexible in structure, the general construction of their mechanical 

 system does not differ appreciably from that which prevails in the 

 fully developed portion of the organ. The difference between the two 

 stages of development consists rather in the quality of the material 

 employed for the construction of the skeletal system : for reasons 

 which have been explained in detail in a previous section (Section A. 3 ), 

 bast is here replaced by collenchyma : this tissue, which is usually 

 developed in the form of subepidermal plates or ribs, thus represents a 

 temporary skeleton, analogous to the scaffolding that surrounds a half- 

 built house. In a number of Monocotyledons (Tradescontia, Dioscorea, 

 Tamus) and herbaceous Dicotyledons (Cucurbitaceae, Umbelliferae), 

 it supplements the true stereome throughout the first season ; in 



