THE STKUCTUEE OF TIMBEK. 271 



pare this division of cells to the bisection, quartering, and so 

 forth of an orange. We may cut up an orange into any number 

 of parts, but we do not thereby increase the quantity of material 

 with which we are dealing. Similarly in the case of young 

 vegetable cells. Cell-division is entirely responsible for increase 

 in numbers, but we must look elsewhere for increase in size and 

 increase in weight. 



To return, then, to the extreme apex of the young growing 

 shoot, we have there cell-division going on energetically, and 

 especially in the front or upper margin of that region. In the 

 lower portion cell-division is not so active, that is to say, the 

 young cells show less disposition to re-divide, or in other words, 

 they show a greater tendency to maintain their individuality. 

 This then introduces us to the second region of our shoot, which 

 lies below the zone or region of cell-division, and which is desig- 

 nated the zone of cell-elongation. There is no sharp line of 

 demarcation between these two regions, that is to say, we have 

 cell-elongation taking place in the zone of cell-division, and 

 cell-division taking place in the zone of cell-elongation, but for 

 the most part the cells are dividing above and elongating below. 



In the zone of cell-division we noted that the cells were prac- 

 tically full of protoplasm, that they were more or less spherical 

 in shape, and that they had very thin walls consisting entirely 

 of that most elastic substance called cellulose. In the second 

 zone, namely, that where cell-elongation is taking place, we find 

 that the cells differ somewhat in appearance from those in the 

 first. The main points of difference are that they are very much 

 longer than broad, and that they are not nearly full of proto- 

 plasm. The difference in shape is accounted for by the passage 

 of water through the cellulose walls into the inside of the cells, 

 and the consequent internal pressure that is developed. This 

 osmotic pressure, as it is called, causes great distension of the 

 cell-walls and results in the cells, which were originally roundish, 

 becoming oblong in shape. So far, however, the character of 

 the cell- walls has not altered. They are still composed of thin 

 elastic cellulose, and it is for this reason, in fact, that any dis- 

 tension is possible. 



