496 Minnesota Plant Life. 



bends towards the light, and there may be enlargement of cells 

 on the convex side, or contraction of cells on the concave side, 

 or both ; yet such a curvature should scarcely be called growth 

 of the organ. Growth of a tissue or organ might perhaps be 

 defined as permanent enlargement arising through growth of 

 the component cells. 



There are ordinarily distinguished three important stages in 

 the growth of an organ. These are: i, the stage of cell- 

 multiplication; 2, the stage of cell-enlargement; 3, the stage 

 of cell-differentiation. A mature organ commonly consists of 

 very many more cells than would be found in the same organ 

 when immature. But since all cells arise from preceding cells 

 and never, so far as known, in any other way, whatever, it is 

 evident that there must have been a general division of pre- 

 existent single cells into cell-groups. This leads to an inquiry 

 into the nature of organ-rudiments. It may be stated broadly 

 that the rudiment of any plant organ is either a single cell or a 

 group of cells. The stems and leaves of a moss or of a liver- 

 wort, the stems and roots of ferns, the spore-cases of "true" 

 ferns and, perhaps, even the stems of pine trees, are examples 

 of organs arising from single-celled rudiments. Of organs 

 arising from groups of cells there might be mentioned the roots, 

 stems and leaves of the higher seed plants, the spore-cases of 

 club-mosses, the spore-cases of seed-producing plants and, in 

 brief, organs generally in higher forms. 



The tip of a growing organ is commonly occupied by a little 

 group of cells in process of division. Thus the tips of roots, 

 the tips of buds inside the covering of overlapping scales, and 

 the tips of very young leaves, will be found to consist of small, 

 thin-walled cells, packed full of living substance, and each 

 capable of dividing into two cells by the formation of partition 

 walls. In such rudimentary areas the cells are at first very 

 similar, but before long some cells of the mass begin to assume 

 shapes and structures that are distinctive. Close behind the 

 growing tip of a young root the skin-cells will have begun 

 to flatten and the cells of the central conduction-path will have 

 begun to elongate. These differences and many others be- 

 come progressively more marked and in older parts of the 

 root each tissue the skin, the wood, the pith, the reservoir- 



