246 GROWTH 



the rapid cessation of growth followed by withering of the whole 

 plant. 



(iii) Food is essential for the construction of the protoplasm 

 and cell-walls of the growing parts. 



(iv) Oxygen is necessary for the process of respiration without 

 which all vital functions cease. 



(v) Light. — The various members of a plant's body grow more 

 rapidly in feeble light than when they are strongly illuminated : 

 that is, light retards growth. 



When grown in darkness for a considerable time plants 

 become peculiarly modified, in which condition they are said to 

 be etiolated. 



Among dicotyledons the internodes of the stems of etiolated 

 specimens are abnormally elongated and much more slender 

 than similar parts grown under ordinary conditions of day 

 and night. Their cells are larger than usual and the cell-walls 

 remain thin ; the stems in consequence become weak and 

 are unable to maintain a normal erect position. The whole 

 plant contains more water proportionately to its size and the cell- 

 sap is usually more acid than that of normally grown plants. 



The leaves of etiolated dicotyledons do not develop but 

 remain small and scale-like and as the chlorophyll does not 

 develop in the plastids the whole plant appears pale in colour. 



A few stems such as those of iris and onion, and the 

 hypocotyls of many plants, such as the bean, which ordinarily 

 grow in the dark, do not exhibit the peculiar phenomena 

 of etiolation, nor are the leaves of iris and other similar 

 rhizomatous and- bulbous monocotyledons dwarfed when grown 

 in darkness. 



The development of the flowers of plants goes on in darkness 

 much the same as in the light. 



Ex. 145. — Sow two sets of peas, beans, mustard, and barley in pots, 

 and allow them to germinate. When the young plants just appear on the 

 surface of the soil, place one set of each in a light situation but not in the 



