PROTOPLASM: AVHAT IT IS, HOW IT MAINTAINS PLANT-LIFE. 79 
If the nucleus be at the side of the cell, it has been seen to travel 
across, dividing in its course and laying down the plate in its passage 
from one side to the other. If an elongated tube be required, such as of a 
sieve-tube or a liber fibre, the nucleus repeatedly divides without laying 
down any cell-plates, till the cell be long enough, when the nuclei become 
absorbed. Similarly in the formation of endospermal tissue, within the 
€mbryo-sac, the nucleus may divide till there are numerous ones of suc- 
cessive generations ; but subsequently cell- walls are formed between 
them. 
Conversely, to form vessels, large drum-shaped cells are piled one 
upon another, but by their partitions becoming absorbed they form long 
and large tubes, such as form the holes seen in a piece of oak-wood. 
When the two daughter cells are formed, fine protoplasmic threads are 
left passing through the new cell-plate, so keeping up a connection with 
the adjacent cell. This protoplasmic continuity is now believed to be 
universal throughout all active and living cells of the entire plant. It is 
thus seen that the nucleus is the most important body in the formation 
of new cells and tissues. Though in the dividing stage the cells are 
much alike, they soon acquire the forms and dimensions required in the 
various tissues and organs in which they occur. 
Another function which special nuclei undertake to perform is that 
involved in the process of fertilisation of^ the germ-cell in the ovule by a 
nucleus of the pollen-tube. We must consider the chief points of con- 
struction of a pollen grain and of an ovule to understand what takes 
place in the formation of the embryo in the seed, which the ovule 
becomes. A pollen-grain consists of two spherical skins ; the outer, 
which is often sculptured in various elegant forms, is called the " extine" ; 
the inner and thinner one is called the " intine." When the grain 
reaches the viscid stigma it opens at definite places, and the intine pro- 
trudes, at first as a pouch. This being stimulated into growth by the 
juices of the stigma, it penetrates the style, growing on till it reaches 
the ovule. This it enters (being led on to the right place by the " con- 
ducting tissue" of the style, &c.) by the orifice called the micropyle," 
and applies its blunt end upon the surface of the embryo-sac. 
Now let us turn to the ovule. This usually consists of a solid central 
body surrounded by two investing skins excepting at one end, where they 
eave the space called the micropyle, mentioned above. Within the 
central body is one cell larger than the rest. This is the embryo -sac. 
Its nucleus behaves quite differently from an ordinary cell-nucleus, in 
that it divides successively into eight nuclei, of which four are found at 
•one end and four at the other of the embryo- sac. One of each of these 
two groups travels to the middle, and they there coalesce to form the 
origin (after impregnation) of endospermal tissue. 
The three upper nuclei, just below the position of the micropyle, are 
all capable of being impregnated and becoming embryos ; but usually only 
one of them does so. This one is called the oosphere or germ-cell. 
When the pollen-tube has arrived at and entered the micropyle, 
absorption takes place both of the tube and of the embryo- sac, where they 
are in contact, so that the two pollen -nuclei can travel through. One of 
them, the sperm-cell, fuses with the germ-cell to form the embryo ; the 
