390 XXVIII. THE ANDRCECIUM OF ANGIOSPEBMS. 



a delicate outer layer beset with spines ; next a delicate " rodlet 

 layer," which corresponds with the dots seen in surface view (Fig. 

 142, A) ; and a thick homogeneous inner layer, forming convex 

 projections inwards. The intine is swollen under the exit-pores, 

 But elsewhere forms a delicate membrane. If such a section is 

 treated with chlorzinc-iodine, the outer layer of the exine and the 

 spines scarcely stains at all, the thickening layer of the exine stains 

 yellow-brown, and the intine blue. The intine in this case is 

 relatively rich in cellulose. The contents of the pollen-grains 

 swell and colour violet, owing to the presence of starch grains, 

 which swell and stain. The unstained as well as the stained 

 sections, provided they are extremely thin and strongly magnified, 

 show us that the exit-pores of the exine are closed externally by 

 a very delicate membrane which passes over them. In order to 

 obtain the best possible sections of pollen-grains, capable of use 

 for the highest microscopical purposes, and of staining for cellu- 

 lose and pectin, it is necessary to embed, the material in paraffin, 

 and cut with a microtome, after the method described in Chapter 

 XXXII. 



If we examine the stigma of an older flower of Malva crispa 

 under the simple microscope, or with a strong lens, many pollen - 

 grains will be found upon it. On the side turned towards the 

 stigma these will have produced numerous pollen-tubes. If such 

 a grain, the pollen-tubes of which are still short, be removed and 

 examined, we can readily determine that the pollen-tubes come 

 from the canals or exit-pores of the exine (Fig. 142, C). This 

 is shown still more beautifully in optical section, after the grain 

 has been made transparent in carbolic acid (Fig. 142, Z>). 



Pollen of Cucurbita. The large pollen-grains of species of 

 Citcurbita have always been specially noticeable for the valves 

 which close the places of egress in the exine (Fig. 143, A). In 

 water, yellow oil-drops come off from the surface of the exine, 

 the grains soon evacuate their contents, and the structure of the 

 membrane then becomes clear. The exine is studded with regu- 

 larly-distributed large spines, and between them very numerous 

 minute ones. The exit-places are round, the valve is lifted up, 

 either on one side or altogether, by the papilla-like bulging of 

 the intine. The valve has the structure of the surrounding exine, 

 and bears one or more of the large spines. Very good figures are 

 obtained in oil of lemon, less useful in oil of cloves. On the other 

 hand, the figures in chloral hydrate are to be preferred to those in 



