270 



REPRODUCTION 



generally oval or spherical bodies of a yellowish colour. The 

 exterior of the grain usually consists of a stout cuticularised 

 cellulose coat — the exine — 

 often elaborately ornamented 

 with spiny, wart-like, or net- 

 like thickened markings ; here 

 and there more or less de- 

 finitely arranged, thinner areas 

 are visible on the coat. Lining 

 this outer protective covering 

 is a delicate transparent cellu- 

 lose membrane — the intine 

 (Fig. 97). The interior of the n 

 grain is filled with cytoplasm, 

 in which are present two nuclei, 

 representing two cells, between , „ „ . . 



^_ ^ . J. .J. Fig. 97. — I and 2 Pollen-grains of a species 



which there is no dividing of Hly, with netted exine, on which small drops 



/. i_ / \ of oil are visible. 3. Section of a pollen-grain : 



cell-wall. One of them \g) a exine ; l intine ; v nucleus of the vegetative 



. jj . cell ; g; nucleus of the generative cell. 4. Ger- 



IS the generative cell or male minatingpolIen-grain;/^pollen-tube;j'nucleus 



. 77 .1 iu / \ of the vegetative cell ; ^^ two nuclei produced 



reprOdUCtWe cell, the other (^) by division of the nucleus of the generative 



being termed the vegetative 



cell of the pollen-grain. Within the cytoplasm, starch, sugar, 



oil and other food materials can often be recognised. 



When a pollen-grain is placed in a weak solution of sugar, and 

 kept at a suitable temperature, it absorbs water, and emits a 

 closed slender tube-like structure, termed 'Cns. pollen-tube {pt), which 

 grows from the vegetative cell of the grain, and may under certain 

 conditions attain a length of several millimetres. The pollen-tube 

 is a protrusion of the intine, and makes its way through the 

 specially thin or otherwise modified places in the exine of the 

 grain. 



During the germination of the pollen-grain the two nuclei 

 present in it travel into the pollen-tube ; the nucleus of the 

 vegetative cell ultimately becomes disorganised and disappears, 



