388 XXVIII. THE ANDROZCIUM OF ANGIOSPERM8. 



the tissue of the style. With magenta the contents of the pollen- 

 grains and tubes will commonly stain deeply, the tissue of the 

 style only very slightly ; and hence the pollen-tubes can be 

 traced with great ease, and often in large numbers. Similarly 

 iodine can be used, and will demonstrate the usual presence of 

 considerable quantities of small starch grains in the stylar tissue 

 which the pollen-tubes are traversing. 



Later in this chapter we will consider the question of the 

 artificial culture of pollen- grains. 



Pollen of Malvacece. Still a few other pollen-grains of specially 

 characteristic form may be considered. The Malvaceae are distin- 

 guished by remarkably large pollen-grains ; we examine those of 

 the Hollyhock, Althcea rosea. In water these appear globular, 

 opaque, studded with colourless spines. They become very 

 beautifully transparent in carbolic acid and in chloral hydrate, 

 much less so in oil of cloves, still less in oil of lemon, all of 

 them media which are used in such cases to make objects trans- 

 parent. The preparations are best in carbolic acid, so that we 

 will use this. Their surface view shows us that the colourless 

 exine is studded, at approximately equal distances, with large 

 pointed spines. Between these are scattered others, short, blunt, 

 of variable thickness. 1 Eegularly-distributed circular openings, 

 appearing rose-coloured, traverse the exine, the basal surface of 

 which is finely punctate. The contents of the pollen -grain appear 

 uniformly finely granular, the nuclei are very difficult to dis- 

 tinguish. The optical section of the grain shows us clearly the 

 form of the large and small spines, and of the canals penetrating 

 the exine. An exceedingly delicate intine can be traced as a 

 boundary to the contents ; it bulges a little/ papilla-like, into 

 the exit-pores of the exine. In concentrated sulphuric^ acid 

 the exine is quickly stained red- brown, and its structure is then 

 very clearly shown. 



The pollen-grains of most other Malvaceae resemble those 

 of Althcea. In Malva crispa, a hardy annual species not infre- 

 quently cultivated, for example, the pollen-grains are shaped 

 just as in Althcea, excepting that the spines on the membrane 

 are all alike ; between the spines lay scattered the exit-pores or 

 canals ; the membrane appears besides finely punctate. 



1 A third form can be occasionally found in which a short spine is 

 situated upon a hemispherical, dome-like base. [ED.] 



