CHAPTER IX.
GROWTH IN SEEDLINGS
When once the young plants start growing under suitable conditions they steadily get bigger. At first sight they appear to grow equally all over, stretching out in each direction as indiarubber does when it is pulled. Let us try to find out whether this is actually the case.
Fig. 28. A Bean seedling: A, with divisions marked on root and stem; B, after further growth, showing where most of the stretching has taken place.
Take a well-grown straight seedling and measure off along its stem and along its root, beginning from the tip, distances 1 or 2 mm. apart, marking them with a fine brush and waterproof ink. Take care not to injure the plant, and also not to make the mark blurred or too big. Draw the plant showing the marks on it as accurately as you can, and make the drawing exactly life-size. Grow it in damp, but very loose sawdust, so as not to rub off the marks, and after one or two days take it out and compare it with your original drawing.
You will find that the whole plant is bigger than when you first drew it. Look carefully at the marks on root and stem, and you will find that they are not all the same distance apart, as they should be if the plant had grown equally all over. The marks which are widest apart are those just behind the tip of the root and below the top of the stem, thus showing that there has been much more growth in these two regions than in the rest of the stem or root (see fig. 28). If you repeat this often with many plants you will find that these are the actively growing parts of the stems and roots; the individual leaves, of course, are also growing. Thus we see that growth is not a simple stretching of the whole, but that there are two definite regions where it is specially active. That of the stem and first root carry on the growth in opposite directions, as we noticed before (see p. [11]), the normal stem growing up into the air and the root down into the soil.
Fig. 29. A, Bean seedling planted upside down. The root has bent right over and is growing vertically down. B, later stage of the same. The shoot has bent up.