HISTOLOGY OF THE EHIZOME. 



77 



house in which matter and energy are stored mainly in the 

 form of starcli, C C H 10 O 5 and in which active chemical changes 

 take place. The cells are thin-walled and soft, and are rather 

 loosely joined together, leaving numerous intercellular spaces 

 (Figs. 39, 40). They contain protoplasm and nucleus, and very 

 numerous rounded grains of starch. This starch is stored up by 

 the plant during the summer as a reserve supply of food just 

 as hibernating animals store up fat in their bodies for use during 

 the winter. Accordingly, it increases in quantity during the 

 summer and decreases in the spring when the plant resumes 

 its growth, before the leaves are unfolded. The parenchyma 

 has also the function of conducting various substances (especially 

 dissolved sugar) through the plant by diffusion from cell to cell. 



The sclerotic parenchyma and sclerotic prosenchyma (Figs. 

 36, 37) are dead, and hence play a passive part in the adult 

 vegetal economy. The former co-operates with the epidermis ; 

 the latter probably serves in part to support the soft tissues, and 

 to some extent affords a channel for 

 the conveyance of the sap. The 

 sap, however, does not flow through 

 the cavities, but passes slowly along 

 the substance of the porous walls. 

 The cells of both these sclerotic 

 tissues have very thick, hard, brown 

 walls, perforated here and there by 

 narrow channels. The cells of the 

 parenchyma are prismatic or polyhe- 

 dral ; those of the prosenchyma elon- 

 gated, and pointed at their ends. In 

 both, the protoplasm and nuclei dis- 

 appear when the cells are fully 

 formed. Towards the apical buds 

 both fade into ordinary fundamental 

 parenchyma. 



Fibro-vascular System. Tkzfilro- 

 vaseular 'bundles (p. 73) are long 

 strands or bands of tissue which 

 appear in cross-section as isolated 

 spots (Fig. 35). The bundles are not really isolated (p. ), 

 however, but join one another here and there, forming an 



O 



FIG. 38. (After Sachs.) View of the 

 rhizome, which is supposed to be 

 transparent so as to show the net- 

 work of the upper fibre-vascular 

 bundles. I, a leaf. 



