GROUP IV. — PHANEROGAMIA. 



435 



eventually developed; or, again, areas are marked off here and 

 there which come ofF like lids under the pressure of the developing 

 pollen-tube (Fig. 282). 



The development of the microspores has already been dealt 

 with in general (see p. 125), so that it will be only necessary 

 4iere to mention certain special points. In the first place there 

 is an exception to the rule that each mother-cell gives rise to 

 four microspores; in Asclepias each mother-cell developes directly, 

 without division, into a cell of the pollinium, each such cell 

 being, at any rate physiologically, equivalent to a microspore ; 





Fro. 293.— Pollen-grains of Malva crispa. A Grain seen from surface ; B section of wall, 

 showing the exine with its alternate spines and pores, the latter closed internally by the 

 delicate innermost layer of the exine ; C germinating pollen-grain with pollen-tubes ; D the 

 same in section, showing the protrusion of the pollen-tubes thronghthe pores. (A, B, D 

 x640; Cx 240: after Strasburger.) 



apparently the same is the case in Zostera where, however, the 

 microspores are isolated. The mother-cells of the microspores 

 either remain coherent, during the development of the microspores, 

 or (as in many Monocotyledons) they become free and float in 

 the granular fluid, derived from the disorganisation of the tapetum, 

 which fills the pollen- sac. The walls of the mother-cells usually 

 become very much thickened, especially in the planes of the future 



