BY W. J. BYRAM. Vll 



are again curiously altered in shape and have taken a stellate 

 formation, and the cells extending the whole length of the 

 slender stem impart to it the qualities of strength combined with 

 flexibility and lightness ; but if a single cell is selected and 

 examined, it will be seen that it still to all appearances answers 

 to the description of an enclosed sac or vesicle. The examina- 

 tion of the vessels of plants led Schleiden to the conclusion that 

 they are the derivatives of a series of cell walls. If we cut a 

 thin longitudinal section from the stem of a fern we see a ladder- 

 like structure, consisting of what are known as scalariform 

 vessels. In these the old cell walls lie in a series close together, 

 and give rise to a vessel whose surface presents the appearance 

 of a series of steps. Another form of cell derivative is found in 

 the spiral vessels where the former cell walls have coalesced 

 to form a lengthened spiral. Leaving the tissues and vessels 

 of the plant, and turning to its reproductive elements, 

 we pick out a few of the pollen grains and ex- 

 amine them under a sufficient magnifying power. We 

 may take as an exainple the pollen-grains of the cotton 

 magnified about 50 diameters. Here we have the biological 

 units no longer united to constitute a tissue or aggregate, 

 but detached and in the form of separate cells adapted to the 

 special function of fertilising the germinal vesicle in the ovary. 

 If we now descend in the scale of plant life and examine 

 the humbler forms, such as the salt and fresh water algae, we 

 find them still presenting the same appearance, for they consist 

 of cell expansions, that is, an aggregate of units, each one the 

 counterpart of the other united together in the form of a frond, 

 as in many of the sea weeds, or filament, as in many of the 

 fresh water forms. If we descend still further we come to 

 minute plants like the desmids, each of which consists of one 

 cell only. Here again we have reached detached units, as in the 

 case of the pollen cells of the flowering plants, but in the latter 

 these units have become differentiated and specialised for the 

 function of reproduction of the plant, while in unicellular plants 

 like the desmids, each cell is a distinct organism, or in other 

 words, is the plant itself. All such observations as these went 

 to show that plants, from the lowest to the highest, are either 

 themselves cells, or are built up of cells, and the modifications 

 or derivatives of cells ; and that the cell is a minute chamber 

 surrounded by a membrane or wall and enclosing fluids. So 

 far the theory was confined to plants, but another German 

 biologist, Schwann, published the results of a series of experi- 



