HOW PLANTS GROW. 



and 2(3, the multiplication of the cells in number. It is by the latter, of course, 

 the principal increase of plants in bulk takes place." 



[We trespass on the next Lesson, to make the illustration still more complete.] 



" 393. Cellular Tissue. — The cells, as they multiply, build up the tissues or 

 fabric of the plant, which may be likened to a wall or an edifice built of bricks, 

 or, still better, to a honeycomb composed of ranges of cells (Fig. 14). 



" 394. The walls of the cells are united where they touch each other, and so 

 the partition appears to be a simple membrane, although it is really double ; as 

 may be shown by boiling the tissue a few minutes, and then pulling the parts 

 asunder. And in soft fruits the cells separate in ripening, although they were 

 perfectly united into a tissue, when green, like that of Fig. 14. 



" 395. In that figure, the cells fit together perfectly, leaving no interstices, ex- 

 cept a very small space at some of the corners. But in most leaves, the cells are 

 loosely heaped to- 

 gether, leaving Fig. 15. 

 spaces or passages 

 of all sizes ; and 

 in the leaves and 

 stems of aquatic 

 and marsh plants, 

 in particular, the 

 cells are built up 

 into narrow par- 

 titions, which form 

 the sides of large 

 and regular canals 

 or passages (as 

 shown in Fig. 15). 



These passages form the holes or cavities so conspicuous on cutting across any of 

 these plants, and which are always filled with air. They may be likened to a stack 

 of chimneys, built up of cells in place of bricks." 



An ordinary size of cell is from g^gth to ^^^jth of an inch ; so that there may 

 generally be from twenty-seven to one hundred and twenty-five millions of cells in 

 the compass of a cubic inch ! Over twenty thousand millions of cells must be 

 formed in a day in the flower stems of the century plant, which grow at the rate 

 of a foot in twenty-four hours, and become about six inches in diameter. 



[Could anything be more lucid or intelligible ? And what is singular, it is 

 new. Vegetable anatomy began with Malpighi and Grew, who arrived at very 

 good views of structure, better than their successors down to forty years ago, or 

 thereabouts. But as to production of cells, or growth by the cell, our present 

 knowledge was opened up by Germans, and more is owing to one still living, and 

 not old (viz : Van Mold, of Tiibingen), than to any one else. He began to pub- 

 lish about twenty-five years ago. 



Only within the last half-dozen years, however, has it been possible to give a 

 simple and clear account like the above of vegetable growth, all based on tho- 

 roughly verified investigations. In this we owe most to Mohl, Schleiden (and 

 earlier, to Mirbel), not to mention a host of very recent vegetable anatomists. 

 In England, it is principally Lindley and Ilenfrey who have put this knowledge 

 into available shape for English readers ; on these Dr. Gray is a decided improve- 

 ment, in the clearness and brevity with which he declares the new truths. — Ed.] 



Fig. 15. — Part of a slice across tlie stem of the Calla JEthiopica, magnified. 



