176 THE PLANT CELL. 



force is known as root-pressure, and is the result both of turgidity 

 and of the damming up of the sap at certain levels in the root 

 and stem until a considerable rise of pressure has been produced. 

 As a result, when transpiration is at a minimum root-pressure 

 is at a maximum, and vice versd* 



By these two methods, then, the dilute solution of salts (raw 

 sap) is either sucked up or forced up through the xylem to the 

 leaves, in the cells of which active assimilation of CO.,, H^O, and 

 nitrogen is proceeding. After the raw sap has been elaborated 

 and added to during photo- and chemo- synthesis it becomes 

 the elaborated sap, and, as such, passes into the thin-walled 

 phloem elements lying on the under surfaces of the leaf- 

 traces in bifacial leaves. It then passes down by means of 

 the large perforations in the sieve-plates of the sieve-tubes, and 

 also laterally by osmosis, and is finally distributed by osmosis 

 into the cortex, young shoots, cambium, medullary rays, and 

 other tissues needing elaborated food-material. Some of this 

 food-material is used at once, but towards the end of the 

 "growing" months a good deal of it is converted into reserve - 

 material for use during the early months of the' succeeding 

 spring. Such reserve-material exists in large quantities in 

 bulbs, tubers, corms, fruits and the phloem, medullary rays, 

 wood-parenchyma, and starch-sheath. The conversion of this 

 stored starch, proteid, &c., during the early spring is due in 

 most cases to the action of enzymes (diastase, synaptase, &c.). 

 Occasionally carbohydrates {i.e., starch - grains) " wander " from 

 cell to cell, being first dissolved by enzymes, and then 

 reconstructed in more remote parts. 



Other forms of reserve food occur, the chief amongst them 

 being : — 



a. Oils and fats in many seeds. 



b. Cellulose (in endosperm). This is dissolved by the enzyme 

 eytase. 



c. Inulin (a carbohydrate) occurring as the splieroids in Dahlia. 



d. Aleurone grains. These are found typically in the endosperm- 

 cells of Ricinus, and are composed of two parts — viz., a CFystalloid of 

 a proteid nature, and a globoid — the latter being a double phosphate 

 of magnesium and calcium. Smaller aleurone grains are also found in 

 Zea mais in the starch-containing cells jviet outside the endosperm. 



* Both root-pressure and transpiration exhibit periodic diurnal fluctua- 

 tions, which are dependent upon a property of the protoplasm known as 

 rhythm or periodicity. 



