2IO THE PHYSIOLOGY OF PLANTS 



leaves, and the latter begin to fade away, beginning near the 

 base of the stem, and die. In perennial plants the material 

 which has been accumulating during the lirst period is used 

 during the second, either in the production of a later shoot 

 (summer shoot) or for the formation of flower-buds which will 

 open in the subsequent spring. In the case of our trees, 

 a great part of the nutritive substance is conducted down 

 from the leaves and stored up in the base of the stem and 

 the main roots, and gradually, when these organs are filled, in 

 more peripheral structures, the lateral branches and the rootlets. 

 This process of storing food material comes to an end when 

 the fall of the leaves ushers in the period of rest. As this 

 partly depends upon the gradual decrease of temperature, the 

 upper portions of the stem, which are exposed to the atmos- 

 phere, may have completely entered the period of rest, while 

 the roots, which lie in the warmer layers of soil, are still con- 

 tinuing their growth in thickness. This activity may some- 

 times last until January, and then only can we say that the 

 plant is entirely at rest. 



Such a period of rest occurs in all plants which last for 

 more than one year. It is either the cold, which stops all 

 vital processes, or, as in tropical regions, the excessive drought 

 of the hot season. The rest due to this latter cause is so 

 important that tropical evergreen parasites belonging to the 

 group of Phanerogams will lose their leaves when they grow 

 on trees which shed their leaves during the dry season. A 

 very excellent example of this phenomenon is Loranthus longi- 

 Jlorus when growing upon Stcrculia, Spondias, Terminalia, or 

 Erythrina. 



Such a period of rest cannot be interrupted with impunity, 

 but it can be shortened by artificial means. Nature itself 

 allows the resting period to be of different durations, accord- 

 ing to the development attained during the preceding period 

 of activity. Tiiis is best seen in the forcing of bulbs, the latter 

 being able to develop very early if the preceding month of May 

 was warm and dry, so that the bulbs could ripen well and early. 

 During- this latent period plants require very little nourishment, 

 and even those green-house plants which retain their green leaves 

 require an infinitesimal amount of water and of nutritive salts. 



