226 HOW PLANTS GKOW. 



While the above changes are going on in the pollen-grain, a 

 change is also seen in the ovule. One of the many cells in the 

 nucleus enlarges at the expense of all the rest, and is called the 

 Ejnbryo-saCy inside which, at its apex, appear two nuclei, round 

 which its protoplasm aggregates. These are "naked" cells — i.e., 

 destitute of cell-wall — and are called the Germinal Vesicles. In 

 these, wx shall see, is the siarting-poi?it of the embryo^ and therefore 

 of the whole plant. 



All is now ready for the fertilization to begin. The time 

 occupied by all these changes in pollen-grain and nucleus varies, 

 from a few hours in Crocus to four or five days in Arum, and 

 some months in Orchids. In Crocus, the length of the " tube " is 

 9,000 times the diameter of the pollen-grain, and yet the former 

 performs its journey in about twelve hours ! The pollen-tube, 

 having reached the ovule, enters it by the micropyle^ or hole left by 

 the double coat just mentioned ; it then applies itself to the apex 

 of the embryo-sac, close to where the germinal vesicles are situate. 

 The tube contains a fluid protoplasm, holding sulphur and fat- 

 globules in suspension ; directly the tube is applied to the embryo- 

 sac, it is supposed that the Fovillce, or contents of the tube, pass 

 through its coat by osmosis into the germinal vesicles. Be this as 

 it may, as soon as the tube touches the vesicles, they are in some 

 way " fertilized," and as the primary result each of them developes 

 a cell-w^all, of course, of cellulose. It matters nothing apparently 

 whether one or both vesicles be fertilized ; one almost always 

 perishes, possibly from deficient nourishment ; still, the tube has, 

 so to speak, " two strings to its bow." The remaining vesicle is 

 our primary cell of the embryo, whose position and construction we 

 have now ascertained. 



ip) Embryonic Growth. 



We have now to trace the primary cell up to the fully-formed 

 embryo, and see the relation of the parts of the embryo to the 

 parts of the adult plant. "We have seen that the vesicles were 

 formed inside the embryo-sac by the process described as free cell- 

 formation. We now see the second kind of multiplication coming 

 into play — i.e., Cell-division. 



The fertilized vesicle divides into two — an upper and lower 

 cell. The upper one is developed by ordinary " division " into a 



