THK STAMKN AND PoLLKN-SAC 249 



The pollen-mother cells are all derived by division from the inner product 

 of the hypodermal cells. They vary in number in different anthers. Caltha 

 gives an average example, in which from 1.4 to lO appear in each transverse 

 section of a pollen-sac (Fig. i (>(,). They are characterised by very thm w.iUs. 

 a dense, non-vacuolated cytoplasm, and a proportionately very lar^e nucleus 

 At first they are closely fitted together, without intercellular spaces (a). Hut 

 presently the pollen-sac distends, and the pollen-mother cells round themselves 

 off, and become suspended individually in a Huitl medium which fdls the spates 

 between them (6). jNIeanwhilc the tapctal cells, from which the fluid proUiblv 

 arises, retain their form, thougli tlicir nuclei often increase by fragmentation. 



l"lG K7O. 



a, b, succussivf stages ul ilL\iln|iiiiL'iit of tin- ciiiitciil^ of ilu' i>oll« 

 ( lou.) !•". (). H. 



As the pollen-niother-cells separate they enter on the tetraddivisiou. Each 

 nucleus divides first into two, and then into four. These rapidly rcj>catc<l 

 divisions are characteristic of all spore-formations, and have an importaiU 

 relation to the constitution of the nuclei themselves. The four resulting 

 nuclei are first enclosed in the single protoplast. But soon each is scparalwl 

 by a partition-wall, and is surrounded by a quarter share of the cytopl.uim. 

 The pollen- tetrad is thus constituted. The four cells are still endosetl by thr 

 common wall ; but later each cell deposits a special wall round itself. The 

 common wall, which was of a mucilaginous character, is then dissolvctl, \*itJi 

 the result that the four cells become dissociated as indei>endenl jxillen -grains 

 (Fig. 194, i. ii.). iMeanwhilc the single nucleus of each has tlivided. K>^>nK '*>« 

 two nuclei present in the mature grain (Fig. i<>j. iii- iv.). 



This description of the developnient sern in Caltha applies for the develop- 

 ment of the pollen in all ordinary Dicotyledons. In .Monocotyledons the 



