PRELIMINARY liVQUIRV INTO THE NATURE OF THE CELL. 3 



In their earliest state the cells of the waod and cork of trees show also 

 conditions of development which correspond essentially to those represented in 

 Fig. I. In these cells, however, a new condition foMows very soon after the 

 appearance of the cell-sap; the protoplasm containing the nucleus disappears, 

 leaving the cell-wall filled either with air or with water. Older wood and cork 

 when completely formed thus consist of a mere framework of cell- walls. 



But now arises an important difference between the behaviour of those cells 

 which enclose a protoplasmic body, and of those from which it has already dis- 

 appeared. The former only can grow, develop new chemical combinations, and, 

 under certain conditions, form new cells. The latter are never capable of further 

 development ; if they are wood, they are of service to the plant only from their firm- 

 ness, power of absorbing water, and from their peculiar form; if cork, they form 

 protecting envelopes which surround the living succulent cellular tissue. 



Since then no further process of development can take place in the cells 

 which no longer contain protoplasm, it may be concluded that the latter is the 

 proximate cause of growth. We shall see in a future paragraph that the de- 

 velopment of each cell begins with the formation of a protoplasmic body, and that 



i.^— > 



FIG. 2.— Sexual reproduction of Fucks vesiculosus ; A cellular filaments bearing antheridia ; B spermatozoids ; / Oogonium, Og with 

 paraphyses / ; // the exterior membranes of the oogonium is split, the inner 2 protrudes, containing the ova; /// an escaped ovum, with 

 spermatozoids swarming round it ; A' first division of the fertilised ovum ; IV 3. young Fucus resulting from the growth of the fertilised 

 ovum (after Thuret, Ann. des Sci. Nat. 1854, vol. ii). (B X330; all the rest X160.) 



the cell-wall is also generated from it ; but the relation of the protoplasm to cell- 

 formation is still more strikingly conspicuous in those cases in which it continues 

 its life for some time as a naked sharply-defined solid body, and only at a later 

 period clothes itself again with a fresh cell-wall, and again takes up cell-sap 

 within itself. We have an excellent example of this in the reproduction of the 

 Fucace^. On the fertile branches of these great marine Algae, of which we may 

 take Fiiciis vesiculosus as an example, large cells are formed in peculiar receptacles, 

 the Oogonia (Fig. 2, /, Og) ; the space enclosed by the cell- wall is densely filled with 

 fine-grained protoplasm, which at first presents a homogeneous mass, but at last 



B 2 



