84 Practical Plant Biology. 



cellulose tube. Closer examination shows that the green colour 

 is actually confined to very minute ovoid or lens-shaped chloro- 

 plasts embedded in a comparatively thick layer of protoplasm, 

 which forms a lining to the cell-wall. This lining extends through- 

 out the whole plant and out into the branches, where it thickens 

 and completely fills their extreme ends. The chloroplasts are 

 limited to the outer zone of this layer and are most closely 

 packed in the ends of the branches. There are no chloroplasts 

 in the protoplasm of the rhizoids. The chloroplasts are very 

 small and appear quite homogeneous, having no pyrenoids. In 

 the same zone and intermingled with the chloroplasts one sees 

 considerable numbers of bright globules varying in size from very 

 minute specks up to ones which have a diameter twice or three 

 times that of the chloroplasts. Chemical tests show that these 

 are droplets of oil. Although when they are artificially liberated 

 from the plant they seem practically colourless, they appear 

 greenish while they are still among the chloroplasts and illumin- 

 ated with the light transmitted through them. The axis of the 

 tube is occupied by a large watery vacuole which extends out 

 into the branches. In unstained specimens no structures can be 

 made out in the inner zone of protoplasm but when stained 

 suitably many very minute nuclei are seen in it. 



Is this plant of Vaucheria to be regarded as a single cell or as 

 multicellular ? Evidently our answer to this question will depend 

 on our definition of the word cell. Robert Hook gave the name 

 of cell originally to the minute box-like chambers out of which 

 a piece of cork is built. He likened them to the cells of a honey- 

 comb and called them by the same name. Evidently he regarded 

 each cavity enclosed by the cell-wall as a single cell, and judged 

 by this definition Vaucheria would be a unicellular plant. On 

 the other hand, it is now well known that Hook only saw the 

 dead remnants of cells in the cork, which when alive contained, 

 like the vast majority of cell-walls, a mass of protoplasm. This 

 mass in its living state is differentiated into nucleus and cyto- 

 plasm. The cell-walls are formed by the activities of the proto- 

 plasm and many instances are known in which these masses of 

 cytoplasm with a nucleus remain for the whole, or part, of their 

 lives without a cell-wall. Hence the wall has come to be regarded 

 as an unessential part of the cell, the essential parts being the 

 nucleus and cytoplasm. You have already seen in the multi- 

 cellular Volvox that although the cells forming the sphere are 

 each enclosed in their cell-walls, yet the cytoplasm of all the cells 

 is continuous, by means of the protoplasmic connections, through- 



