INTERCELLULAR WATER-STORAGE. STARCH 409 



D. STORAGE OF WATER IN INTERCELLULAR SPACES. 



inn 



In vegetative organs, the intercellular spaces almost always serve 

 for ventilation ; in a few cases, however, they are utilised for storage of 

 water. This very exceptional condition is exemplified, according to 

 Schimper, by the epiphytic Aroid Philodcndron cannaefolium. The 

 swollen spindle-shaped petioles of this plant contain large intercellular 

 spaces which, during wet weather, are entirely filled apart from 

 minute air-bubbles with a liquid of mucilaginous consistency. When 

 the external water-supply fails, the liquid gradually disappears from 

 these intercellular spaces, being transferred, as Schimper's experiments 

 show, to the transpiring leaf-blades. 



It is more usual for cell-layers containing numerous intercellular 

 spaces to be employed for purposes of water-storage in pericarps and 

 seed-coats. The hard central portion of the fruit of Poterium spinosum, 

 for instance, is enveloped in a thick layer of tissue which in the dry 

 state is full of air. According to Klebs, this layer consists of "stellate" 

 parenchymatous tissue containing numerous large intercellular spaces ; 

 when the fruit is wetted, both the cell-cavities and the intercellular 

 spaces of the stellate tissue become filled with water. Heinricher 

 states that, in Adlumia cirrhosa, the inner epidermis of the pericarp is 

 converted into an open framework with wide intercellular meshes. 

 The cell-walls are lignified and provided with numerous pits ; when 

 this layer is wetted, all the interstices of the meshwork intercellular 

 spaces as well as cell-cavities become filled with water. 



III. STORAGE OF PLASTIC SUBSTANCES. 



A. THE VARIOUS CLASSES OF RESERVE-MATERIALS. 



1. Non-nitrogenous reserve materials. The non-nitrogenous reserve 

 materials that are deposited in storage-tissues, consist either of carbo- 

 hydrates or of fatty oils. Reserve carbohydrates frequently occur in 

 the solid form of starch or cellulose ; some of them, such as inuline or 

 the various sugars, are contained in solution in the cell-sap. 



Starch 191 generally takes the form of polyhedral or rounded granules, 

 which vary in size from macroscopic to almost ultra-microscopic dimen- 

 sions. The shape of the individual grains varies greatly in different 

 plants, but is usually constant for any given species. The starch-grains 

 of the Pea, the Bean, and other Leguminosae are ellipsoidal, with a 

 central hilum ; those of Wheat and Rye are lenticular, while those of 

 the Potato are ovoid in shape and excentric. These rounded shapes 

 only occur where the starch-grains have room to expand freely in all 

 directions, and thus do not interfere with one another's development. 



