406 STORAGE SYSTEM 



mucilage expand at their distal ends and finally become confluent with 

 one another. The innermost (tertiary) thickening layer of the lateral 

 walls takes no part in the swelling, but splits up, in each cell, into one 

 or more spirally-twisted ribbons, which are drawn out by the 

 expanding mucilage-tubes. The final result is a solid mass of 

 mucilage containing numbers of these spiral ribbons with their coils 

 drawn widely apart. The presence of such fibrous structures in 

 mucilage-layers is by no means a peculiarity of the nutlets of Salvia ; 

 a similar arrangement prevails in the seed-coats of a great many 

 plants, representing a variety of Natural Orders (e.g. in Ocymum, 

 Senecio, Collomia, Gilia, Ipomopsis, Polemonium [according to Unger]). 

 The spiral fibres, therefore, presumably have some definite function to 

 perforin ; in all probability they constitute a sort of skeleton or 

 framework, which holds the mucilage together and thus prevents it 

 from dissolving away too rapidly. 



In addition to their principal function of preventing desiccation 

 and of regulating the water-supply, such superficial mucilage-layers 

 also assist in attaching the seed or fruit to the substratum. 



C. WATER-STORING TRACHEIDES. 1 " 9 



There is a special type of water-storing element, which is dis- 

 tinguished by the absence of a living protoplast, and by the presence 

 of stiffening arrangements in the cell-wall, similar to those that are 

 characteristic of vessels and tracheides. The reservoirs in question 

 are filled with water injected into them from adjoining vascular 

 bundles or from living parenchyma. When they are emptied, they do 

 not collapse, like typical water-tissues, but become filled with rarefied 

 air. These elements obviously resemble ordinary tracheides in many 

 respects, though they differ markedly from the latter in their function 

 which is not conduction but storage and in certain structural 

 features correlated with this difference in function, such as their larger 

 calibre and their frequent approximation to an isodiametric form. As 

 a matter of fact, the cells in question have been termed storage- 

 tracheides by Heinricher (Vesque's name of reservoirs vasiformes has 

 the same significance). 



In many plants, storage-tracheides only occur in the distal expan- 

 sions of the bundle-ends ; here the terminal portions of the water- 

 conducting system are transformed into organs of water-storage (Fig. 

 160). The walls of the elements in question are more or less lignified, 

 and are either strengthened with reticulate or spiral thickenings, or 

 else are provided with numerous transversely elongated pits. Phylo- 

 genetically considered, these cells sometimes correspond to terminal 

 tracheides of the vascular bundles, which have expanded and taken on 



