4 GROWTH 



(Somatophytcs) growth results in the production of adult parts and 

 organs, whereas in such asomatic plants as bacteria, Spirogyra, and Oscillaria 

 the products of growth may retain their embryonic character, and remain 

 capable of growing and reproducing new individuals. This is only possible 

 in small and lowly organized plants, for masses of embryonic cells do not 

 possess the strength and rigidity required by large plants, and the embryonic 

 character is usually lost in tissues or organs which undergo pronounced 

 morphological and physiological differentiation. 



Since the somatic parts must die sooner or later *, the preservation of 

 embryonic cells is necessary for the maintenance of the species. Spores and 

 fertilized ova are in fact embryonic cells, and cuttings are able to produce 

 new individuals because certain of their tissues remain embryonic and are 

 able to reproduce missing parts. 



In addition to forming new individuals, Somatophytes also retain 

 embryonic cells and tissues which continually add new parts and organs to 

 the parent plant, frequently until death ensues. These meristematic cells 

 and tissues found at the growing points and along the cambial zones remain 

 continually young, while the cells segmented from them give rise directly 

 or after previous division to new tissue elements. These may either be 

 superposed upon older ones, as when the stem of a tree increases in thickness, 

 or may give rise to new organs, such as the new leaves formed upon a 

 deciduous tree in spring. 



The growth of an annual plant is also the result of the activity of locally 

 preserved regions of embryonic tissue, and in many plants intercalary 

 vegetative zones of longer or shorter duration may be produced in order to 

 attain special ends 2 . 



Owing to this power of continued growth, combined with the fixed 

 habitat, the life of most plants assumes a widely different character to that 

 of a typical animal, which may live long after a permanent adult form and 

 size have been attained. But even here there is no essential difference 

 between the two kingdoms, for sponges and corals simulate plants in their 

 fixed habitat and continuous growth, while in many of the lower animals 

 and plants no definite somatic parts are as yet differentiated. 



Since all living beings are derived from similar pre-existent ones, it 

 follows that the continuity of the embryonic plasma is essential for the 

 maintenance of a particular species. In other words, the consecutive 

 generations are linked together by the embryonic substance to a single 

 rhythmically repeated series. Since the ontogeny of a particular species 

 always begins with the same embryonic materials, it must naturally follow 

 the same path so long as the species remains constant. 



1 On the duration of life in somatic and embryonic cells cf. Sect. 64. 



2 For examples see Goebel, Ent\vickelungsgesch. d. Pflanzenorgane, 1883, pp. 153, 179. 212; 

 Hofmeister, Allgem. Morphol., iS6?, pp. 240, 465. Cf. also Sect. 3. 



