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 i)crha])S be 

 defined as permanent enlargement arising through growth of 

 the component cells. 



There are ordinaril)' distinguislied three important stages in 

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

 multii)lication ; 2, the stage of cell-enlargement ; 3, the stage 

 of cell-differentiation. A mature organ conmionly consists of 

 ver}' many more cells than would be foimd in the same organ 

 when immature. But since all cells arise from preceding cells 

 and never, so far as kno\\n. in any other way. whatever, it is 

 evident that there nmst ha\e been a general division of pre- 

 existent single cells into cell-groups. This leads to an in(|uiry 

 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 lixer- 

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

 ferns and, perhaps, c\-en 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 conunonly occupied by a little 

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

 the tips of buds inside the covering of overlap])ing scales, and 

 the tips of \ery xoung lea\es, will be ftJund to consist of small, 

 thin-wallcd cells, packed full of li\-ing stibstancc. and each 

 capable of di\-iding into two cells b)' the formation of partition 

 walls. In such rudimentar}- areas the cells are at first very 

 sinn'lar, but before long some cells of the mass begin to assume 

 shai)es and structures that are distinctive. Close behind the 

 growing ti]) of n young root the skin-cells will have begun 

 to flatten .-nid the cells of the central conduction-path will have 

 begun to elongate. These differences .and many others be- 

 come progressi\"ely more marked .and in older ]).'U'ts of the 

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



