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A TEXTBOOK OF THEORETICAL BOTANY 



The nuclei usually remain unchanged or somewhat enlarged, but m 

 Arabis they disorganize. Total plasmodial formation is not always achieved, 

 e.g., Knauiia and Cobaea, where the individual cell limits remain distinct. 



The secretion type of behaviour does not start with the dissolution of 

 the cell walls (Fig. 1269). Instead, the cells collapse and there is a gradual 

 disappearance of their contents, the walls going last of all. Solution occurs 



Fig. 1269. — Lonicera coeruleo. Plasmodial tapetum. Right: the tapetal cells have lost their 

 walls and are changing in shape. Left : completed plasmodium surrounding the pollen 

 grains. (From Ji'el, "jf.j- tciss. Bot.'\ 56.) 



much later than in the first type, sometimes not until the anther is practi- 

 cally ripe, and usually not before the pollen grains have reached the two- 

 celled stage. Meanwhile the spaces between the grains are filled with a 

 homogeneous colloidal material which can be stained and is presumably 

 secreted by the disorganizing protoplasts of the tapetal cells. It is aug- 

 mented by the dissolution of the mother cell walls. 



The periplasmodial type, although it is found in the Pteridophyta (see 

 under Botrychium in Volume I), is unknown in the Gymnosperms. It may 

 be common throughout certain large systematic groups, e.g., Rubiales, where 

 it has been demonstrated in Rubiaceae, Caprifoliaceae, Valerianaceae and 

 Dipsacaceae. On the other hand both types may occur in closely related 

 genera. For example, Cobaea is periplasmodial and Polemoniiim is secretory ; 

 Lonicera is periplasmodial but Viburnum is secretory. 



The chemical analyses previously quoted show that pollen grains are 

 rich in carbohydrates and fats. Both these kinds of material may be stored 

 in some quantity, the carbohydrates either as starch grains or sugars and the 

 fats as droplets disseminated in the cytoplasm. In Forsvtliia, the flowers of 

 which are heterostylous, it is a remarkable fact that the pollen grains, in the 

 short-styled flowers only, contain 25 per cent, of lactose. This is the only 

 recorded occurrence of this sugar in plants. Furthermore, it is formed in a 

 male cell, whereas in animals the production of lactose is predominantly a 

 female characteristic. 



