U 4 PLANT PHYSIOLOGY 



of long shoots ? It is easily seen that the problem is much more complex, and 

 it would appear to us that a certain condition of the protoplasm is essential for 

 determining the nature of the resulting formations. This condition of the proto- 

 plasm might, on its part, be due to the primary changes in the ' internal con- 

 ditions ', emphasized by KLEBS and indeed there cannot be the slightest doubt 

 that KLEBS admits profound subsequent changes of this kind, and regards them 

 as essential. But this state of the protoplasm might be primary, and it might 

 induce one cell to grow only moderately, and so store up abundant reserves, 

 while it induced another to grow rapidly, and to go on using up the materials 

 brought to it. This condition of the protoplasm is not the specific structure in 

 KLEBS'S sense of the term, but it is the final result of its ' internal conditions '. 

 It would appear to us that this factor had a special significance over other more 

 external factors, such as the concentration of cell-sap, amount of enzymes 

 present, &c., a significance which in KLEBS'S argument is not made sufficiently 

 prominent. 



366, 11. 4-5, for As above observed . . . mode of origin, read It would be 

 very convenient if it were possible to replace the usual classification of repro- 

 ductive organs into sexual and asexual by a better ; for organs such as swarm- 

 spores and sexual cells of Algae, despite their well-known relationship to each 

 other, are grouped into two categories, while, on the other hand, bodies so 

 distinct as swarmspores, propagative bulbs, isolated leaves or twigs, &c. are 

 grouped together as asexual organs. HANSTEIN (1877) has already attempted 

 such a classification, viz. into ' germs ' and ' buds ', and more recently MOEBIUS 

 (1897) has striven to carry out this distinction further by distinguishing these 

 bodies by their mode of origin. 



367, title of lecture, delete note in square brackets. 



11. 11-17, for In endeavouring . . . 1899). read"We will now endeavour to 

 examine more closely into the significance of fertilization. 



1. 34 P. 370, 1. 14, for It is probable . . . stimulus, read The nuclei of ova, 

 as also of spermatozoa, exhibit one remarkable difference from those of vegeta- 

 tive cells. In the development of the sexual cells a special kind of mitosis takes 

 place which has been termed reduction division. The essential features of this 

 mitosis are surprisingly similar throughout the whole organic world, and its 

 principal characteristic lies in the fact that the daughter cells have only half 

 the usual number of chromosomes. In ordinary mitosis longitudinal splitting 

 of the chromosomes takes place, and each of the longitudinal halves passes to 

 a daughter cell, but in reduction division the unsplit chromosomes pass to the 

 daughter cells (comp. STRASBURGER, 1905 b). 



Since the ovum and the sperm cell have only half as many chromosomes 

 as the ordinary cell nucleus, it is reasonable to assume that their individual 

 incapacity for development is bound up with this fact and that, conversely, 

 the development that ensues subsequently to fusion is the result of the reap- 

 pearance of the normal number of chromosomes. This view at first sight 

 appears to be confirmed by the phenomena of Parthenogenesis. By this we 

 mean the development of the egg without previous fertilization (STRASBURGER, 

 1904 and 1907 ; FARMER and DIGBY, 1907 ; WINKLER, 1906). Parthenogenesis 

 has been found to occur in many ferns, species of Marsilia, Alchemilla, Wick- 

 stroemia, and many other plants. 



Many of these have entirely lost the male cells necessary for bringing 

 about fertilization, and in all of them parthenogenesis has become the normal 

 mode of development ; it does not take place merely when, by some accident, 

 fertilization has not been effected. In all these cases where accurate cy to- 

 logical investigations were possible it has been found that the ovum exhibits 



