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CHAPTER II. DISEASES. 

 SECTION I. INTRODUCTORY REMARKS. 



Duration 153. So far as we know at pre- 



of Plant sent, all the embryonic tissue of plants, such as the cambium, 

 which has hot become converted into specialised tissue or 

 organs, is capable of unlimited life, providing the external 

 conditions are such as to allow of its growth. The continued 

 existence of plant life upon the earth in fact depends on 

 this property of embryonic cells. Thus bacteria, the cells 

 of which all retain their embryonic character and power 

 of continued growth and of producing new individuals, possess 

 within themselves the power of unlimited life. In annual 

 plants the death of the plant ensues on the production of 

 the seed. The seeds, however, contain embryonic cells, and 

 the continued existence of the latter renders the mainten- 

 ance of the species possible. Trees, on the other hand, 

 owing to the presence of the embryonic cambium, are 

 often capable of living for very long periods and may reach 

 an age of more than 1,000 years. In a tree considerable 

 portions continually die in the ordinary course of natural 

 development. Thus the root-hairs only live for a short time, 

 large quantities of wood are killed to form heart-wood, masses 

 of external tissue die and become dead bark, the leaves live for 

 a few years only at the longest, sepals, petals and stamens have 

 a very brief life, so that in an old tree the actually living tissue 

 which has arisen from the embryonic cambium and become 

 converted into special tissue, or organs with a particular func- 

 tion to perform, is at most but a few years old. Not only in 

 annuals, but in some plants which do not flower for several 

 years, such as some species of Strobilanthes, Bamboos and some 

 Palms, the production of flowers and seed causes such 

 exhaustion of the plant's vitality that death ensues. In some 

 cases it has been found possible to prolong a plant's existence 

 by artificially preventing the production of flowers, and cases 

 are known .in which annuals have thus been made perennial. 

 Whether on account of internal or external causes, however, 

 so far as we know at present, every individual plant does 

 sooner or later die. 



Definition of 1 ^^ : - ^ n na ture the development 



Disease. of every organism depends on the number and intensity of the 



injurious and beneficial influences, not only of other organisms, 



but also of the non-living environment, which affect its develop- 



