17 



In Dicotyledons the rule is for the four pollen nuclei 

 to be formed before they are divided by cell walls — 

 simultaneous division (Fig. 21). 



HOMOTYPE MITOSIS. 



In this form of nuclear division applicable to tissue or 

 somatic cells, i.e., the ordinary or body cells of the plant 

 and those cells immediately preceding and succeeding the 

 spore mother-cells, and in which there is no stage com- 

 parable to synapsis so characteristic of the prophase in 

 mother-cells. The number of chromosomes is always alike 

 in parent and daughter nucleus ; in cells of the gameto- 

 phyte the reduced number obtains, in the sporophyte cells 

 twice as many.^^ For Lilium the numbers given are 

 respectively 12 and 24.* 



Immediately following the telophase of the previous 

 heterotype mitosis in the mother-cell, the spirem of the 

 daughter cell breaks across (Fig. 22) the prior points of 

 union into chromosomes corresponding in external features 

 with those from which the spirem originated, and are 

 believed to be identical by Allen in Lilium, Gates in 

 CEnothera, also by others. Spindle threads appeal- 

 attached to the chromosomes, which move to the equator 

 of the cell and form the nuclear plate, both together 

 constitute the spindle ending the prophase (Fig. 23). 



Each chromosome now splits longitudinally into two 

 equal halves — equation division (Fig 24). The line of 

 separation is considered to be the same as that seen in the 

 anaphase of the previous mitosis.'', ^^ The chromosomes 

 are now drawn apart and towards the respective poles by or 

 along the spindle fibrils, there they become closely pressed 

 together (Fig. 25) and enter upon the telophase. 



• For numbers in inanj- other plants, see Morphologv of Angiosperms, 

 Pt. 2, 1904, pp'. 81.2. 



