1021 



Fig. 9. Visum album. From a section parallel to the surface, treated with 

 sulphuric acid. The cuticnlar ridges which are seen in Fig. 3 in relation 

 to the cell- wall net, have been loosened by the action of the sulphuric 

 acid. The "hgamcnls of articulation" are dissolved in the sulphuric acid ; 

 a and h are in normal position, at c the ridges have fallen outwards. 



Fig. 10. Rhipsalis Gassytha. From a median longitudinal section through the 

 stem. The stoma in transverse section, the guardcells roughly halved. 

 The section has been somewhat distorted in cutting and preparing, so 

 that the slit, especially in the middle and below, is wider than in the 

 intact plant. 



Fig. 11. Rhipsalis Gassylha. From a section parallel to the surface. The level 

 is that of the central slit. 



Fig. 12. Rhipsalis Gassytha. From a section parallel to the surface. The level is 

 above the guard-cells, the outer slit is seen. 



Fig. 13, Rhipsalis Gassytha. From a section parallel to the surface, placed in 

 an inverted position on the slide. The level is below the guard-cells, 

 the canal-shaped internal air space is seen, which is closed by the two 

 subsidiary cells. 



Fig. 14. Rhipsalis Gassytha. From a transverse section through the stem. The 

 stoma in longitudinal section parallel to the slit, in a direction 

 roughly corresponding to line e in Fig. 1, the preparation is orientated 

 in a corresponding manner to the preparation of Viscum drawn in Fig. 8. 



Leiden, December 1913. Botanical Laboratory. 



Botany. — "The explanation of an apparent exception to Mendel's 

 law of segregation.'' By Miss Tine Tammes. (Communicated 

 by Prof. J. W. Moll). 



(Gommunicated in the meeting of February 28, 1914). 



In experiments on hybridisation in recent years various cases liave 

 been observed in which the numerical proportion of different forms 

 occurring in the second generation does not agree with what might 

 be expected according to Mendel's law. Among these there are very 

 many with respect to which there is no reason to assume that tliis law 

 does not apply, and in the greater number of these cases it has been 

 possible to show the causes of the discrepancy. These causes have been 

 found to be of two kinds. Firstly, there may be deviations which 

 are only the results of mistakes or wrong hypotiieses on the part 

 of the observer. Secondly, there are cases in which the deviations 

 are due to the plant itself. The sources of error belonging to the 

 first class are chiefly as follows. 



1. It may happen that the deviation is the result of making too 

 few observations. 



