I r BOTANY OF THE LIVING PLANT 



. hi ,, ot hcr conditions are favourable. In order to force plants 



rows them in hot-houses, or in frames slightly 

 NVil ' rn . nentative changes in rotting manure upon which 



r ram( i. The rise of temperature in spring and early summer 



„• in the stimulation of growth at that season, while 



i lv the tall of temperature in autumn is at least a contributary 



the period of dormancy in vegetation at large which then sets 



The high temperatures prevailing in tropical regions, combined 



with the generally favourable conditions, result in very rapid growth : 



thus sho : the Giant Hamboo may grow in length over a foot a 



day, a rapidity which can be followed by careful observation with a 



hand lens. 



Exposure to temperatures in the region of o° C. results in growth and other 



\ ital acth ities being arrested, but it does not necessarily kill plants ; neverthe- 



in many plants, such as Tomato, Potato or Dahlia, fatal results may fol- 



I n mi, li i • hough there may be a direct effect of the low temperature 



on the protoplasm, it is the formation of ice in the tissues that contributes 



tii- death of the plant. The ice forms in the intercellular spaces and 



to the withdrawal of water from the vacuoles and protoplasm of the 



lis. The protoplasm is killed by such dehydration, probably because its 



proteins become coagulated. In addition, the development of ice within 



th .-s may cause mechanical injury. By suitable treatment many plants 



hardened," and are then able to withstand exposure to moderately 



low temperatures without injury. Some plants, especially those inhabiting 



polar regions, are so constituted that they can withstand exposure to very 



low temperatures, in some cases as low as -50°C, without being killed. 



The full explanation of this ability to withstand low temperatures is unknown. 



1 xposure of plant organs to low temperatures results in the conversion of 



any starch that may be present into sugars : the sweetening of frosted potatoes 



cample of this. Experiment shows that in the presence of sugars, 



proteins are less easily coagulated by freezing than otherwise. Also, hardy 



plants or organs tend to be relatively low in water content : less ice can 



therefore be produced, while of the water that is present an unusual proportion 



may be bound up as water of imbibition in colloidal substances, and not 



idily subject to freezing. 



Water. 



ill-enlargement is dependent on an adequate supply of water. 

 For normal growth the plant must have access to such a supply: 

 the plant will otherwise be stunted owing to the failure of the cells 

 to attain their normal size. 



Internal Regulation of Growth. Plant Hormones. 



An obviou are in the growth of a plant is that there is co- 



ordination of the growth of its various parts, which is adjusted 



