98 PLANT PHYSIOLOGY 



more quickly than cells in their immediate neighbourhood, both micro- and 

 macroscopically identical. All these various forms of ' primordia ' may be 

 described as ' reserve organs ' (GoEBEL, 1902). It is readily conceivable how 

 by their presence the plant is saved the labour of forming renovations. Hence, 

 when a large part of the apex of the root is cut off, the neighbouring lateral 

 roots take on the function of the primary root, and, in the case of the stem, 

 a number of buds shoot out, which were only waiting this opportunity to do so. 



330, 11. 17-18, for Indeed VOCHTING ... (p. 333). read MIEHE'S experi- 

 ments on Cladophora, described above, have shown us that the polarity exists 

 in individual cells, and we shall meet with further illustrations later on. 



1. 23, after factors read (VOCHTING, 1906). 



1. 39, for 1898-1901, p. 435 read Organography, II, p. 227.) 



331, 1. 4, after (peloria) read That peloric formations may also occur in 

 lateral flowers does not contradict this statement, for lateral members only may 

 be dorsiventral but need not be so. 



Having now become acquainted with a series of examples, let us inquire 

 into the conditions and causes of regeneration. Certain external factors, such 

 as temperature, moisture, &c., are generally concerned in regeneration, and if 

 these conditions be fulfilled we see, in many plants, some organ or other becomes 

 regenerated, while in others only young cells have this capacity, or the capacity 

 for regeneration may be wanting altogether ; these differences in behaviour 

 are specific, and no reasons can be given for them. Apart from this, however, 

 we find that in lower plants generally, where only slight tissue differentiation 

 exists, regeneration is much easier than in higher plants, and in the latter the 

 capacity for regeneration decreases as tissue differentiation increases. Further, 

 in plants with. great powers of regeneration, such as Hepaticae, cells such as 

 those which give rise to root-hairs, or to mucilage cells, are no more able to give 

 origin to the entire plant than are the assimilatory cells. Hence it is probable, 

 though not absolutely demonstrable, that all the cells of the organism when 

 just formed have the power of reproducing the entire body, but that they 

 gradually lose the power the more they become specialized to the performance 

 of definite functions (comp. GOEBEL, 1905). 



The causes of regeneration, which have been recently discussed compara- 

 tively from many points of view (GOEBEL, 1902, 1905 ; KLEBS, 1903 ; MACCALLUM, 

 1903 ; VOCHTING, 1906), are as yet known rather negatively than posi- 

 tively. Thus we may say that the actual wounding does not of itself 

 induce regeneration. An organ may often be very severely injured, e. g. by 

 longitudinal sectionizing, without any regeneration taking place, while con- 

 versely a regeneration results from comparatively slight wounds, and even 

 without any wounding at all. Shoots may arise from the leaves of Begonia 

 or Utricularia not only when these leaves are cut off, but also after the growing 

 point has been removed or rendered inactive. Such inactivity may be induced, 

 mechanically, by enclosing it in plaster of Paris, by inhibiting its activity by 

 chemicals, e. g. an atmosphere of hydrogen, and even by darkening ; regenera- 

 tion has been observed under all these conditions. In many cases all that is 

 needed to induce regenerative structures is to separate the growing organ from 

 the rest of the plant by ringing into the wood, though in the spruce, for example 

 (ERRERA, 1905), this is not enough to induce the formation of an orthotropic 

 lateral branch ; on the other hand, this result may be obtained by sharply 

 bending the terminal branch. 



If the further question be asked, wherein essentially the influence of the 

 growing or active organ lies, how its activity interferes with the development 

 of other organs, it would appear that none of the various explanations offered 

 are satisfactory ; more especially we are not dealing here in general with dis- 



