PRELIMINARY INQUIRY INTO THE NATURE OF THE CELL. 
3 
this time relatively very large (/'). The cell-sap first appears, when the cell is grow- 
ing quickly (B), in the form of drops ( Vacuoles) in the interior of the protoplasm 
{B, s); at a later period these drops usually coalesce, and form a single sap-cavity 
{C, s), which is. enclosed by the now sac-like hollow substance of the protoplasm. 
In their earliest state the cells of the wood and cork of trees show conditions 
of development which correspond essentially to those represented in Fig. i. In 
these cells, however, a new condition follows very soon after the appearance of the 
cell-sap ; the protoplasm containing the nucleus disappears, leaving the cell-cavity 
filled either with air or with water. Older wood and cork thus consist of a mere 
framework of cell-walls. 
An important difference exists between the further behaviour of those cells 
which enclose protoplasm, and of those from which it has disappeared. The former 
only can grow, develope new chemical combinations, and, under certain conditions, 
Fig. 2. — Sexual reproduction oi Ficais vesiculosns ; A branched hair bearing antheridia ; j5 antherozoids ; / an oogonium witli 
paraphyses /; // the exterior membrane a of the oogonium is spht, the inner membrane i protrudes, containmg the oospheres ; III an 
oosphere escaped, witli antherozoids swarming round it; first division of the oospore or fertilised oosphere; IV young plant 
resulting from the growth of the oospore (after Thuret, Ann. des Sei. Nat. 1834, vol. II). (/9 X 330 ; all t'he rest X 160.) 
form new cells. The latter are never capable of further development ; in the case 
of wood, they are of service to the plant only from their firmness, power of absorbing 
water, and peculiar form ; in cork, by forming protecting envelopes which surround 
the living succulent cellular tissue. 
Since then no further development can take place in cells which no longer 
contain protoplasm, it may be concluded that the latter is the proximate cause of 
growth. We shall see hereafter that the development of each cell begins with 
the formation of a protoplasm-mass, and that the cell-wall is also generated from 
it ; but the relation of the protoplasm to cell-formation is still more striking when 
it exists for some time in the free state as a mass of definite form, which eventually 
clothes itself again with a fresh cell-wall, and takes up cell-sap. We have an 
excellent example of this in the reproduction of the Fucacese. On the fertile 
branches of these large marine Algae, of which we may take Fucus vcsiculosus as 
