408 



STORAGE SYSTEM 



cells with narrow pits on their walls. Immediately below this water- 

 tissue lies a single layer of large palisade-shaped storage-tracheides, 



with spiral thickenings which alternate 

 on the two sides of each radial wall. 

 The continuity of this tracheidal layer 

 is frequently interrupted by water-tissue 

 cells, resembling the tracheides in shape, 

 and, like the latter, extending inwards 

 as far as the photosynthetic palisade- 

 layer. The spongy mesophyll contains 

 numerous isodiametric or transversely 

 elongated water-storing tracheides. Fin- 

 ally, there is a second continuous layer 

 of storage-tracheides situated next the 

 abaxial epidermis; like the adaxial layer, 

 it is interrupted, at a number of points, 

 by parenchymatous water-storing cells. 



Most frequently, however, storage- 

 tracheides occur scattered singly through- 

 out the mesophyll. In the genus 

 fieaumuria (Tamariscineae), for example, 

 both the palisade-tissue and the spongy 

 parenchyma of the needle-shaped leaves, 

 according to Vesque, contain numerous 

 " reservoirs vasiformes " which as their 

 shape suggests represent modified pali- 

 sade- or spongy cells. Kriiger has 

 described the storage-tracheides which 

 occur in the mesophyll or in the pseudo- 

 bulbs of certain epiphytic Orchids ( Lvparis 

 Jilipes, Oncidium spp.) ; the cells in ques- 

 tion may be isodiametric or elongated, 

 and are usually provided with spiral 

 thickenings. According to Kny and 

 Zimmermann, the characteristic tubular 

 " spiral cells " in the stem and leaf of 

 Nepenthes, which after active transpira- 

 tion are filled with rarefied air or 

 water- vapour, likewise represent water- 

 reservoirs. The " parenchymatous tracheides " discovered by Eothert in 

 the pith of Cephalotaxus Koraiana are probably of a similar nature ; 

 they are furnished with irregular thickening fibres, and also with 

 bordered pits. 



Fig. 161. 



T.S. through a leaf of Phi/sosiphon 

 Landsbergii, showing water-tissue and 

 storage-tracheides. 



