736 MECHANICAL LAWS OF GROWTH. 



the leaves, which also begin to lengthen at the same time ; the older mature leaves 

 are generally placed on mature internodes. If the internodes are clearly marked 

 out from one another, as is especially the case when the leaves are verticillate or 

 sheathing at their base, each internode forms a more or less individualised whole 

 as soon as it emerges from the bud, and different stages of growth may be distin- 

 guished in it, advancing from below upwards. This may take place in two different 

 ways, according as the uppermost or lowermost part of an internode remains in an 

 undeveloped condition, the other end being completely mature. This zone which 

 continues for some time in an undeveloped state — cell-division taking place actively 

 in it — is more commonly found at the lower than at the upper end of the inter- 

 node (as in Phaseolus), especially when it is enveloped by closely adpressed leaf- 

 sheaths or by a bulb, as e. g. in Equisetacese (especially E. hyemale), Umbelliferae, 

 the bulbous Liliaceae, the haulms of Grasses, &c. If the internodes are not sharply 

 distinguished, as in stems with small leaves and the floral axes of Dicotyledons, the 

 various states of growth which have been described pass insensibly into one another 

 on the stem ; and this is always the case with roots. If leaves when once expanded 

 continue to grow for some time, the process is the same as with flower-stalks or 

 branches ; while the lower portion of the leaf-stalk is fully mature, the upper parts 

 present successively younger or less developed states. The formation of cells 

 finally ceases at the apex and all the parts then become fully mature. This is 

 strikingly the case in Ferns, less so in the pinnate leaves of Papilionaceae or the 

 incised leaves of Araliaceae. 



But very often the activity of the piinctum vegctationis of the leaves lasts for only 

 a short time and its tissue matures while cell-divisions still continue at the base of 

 the leaf, and all the transitional states of growth are to be found between the base 

 and the apex. This occurs, for instance, in the long leaves which grow from 

 the bulbs of Liliaceae and allied Monocotyledons. When a cell-producing zone 

 of this kind occurs at the base of an internode or of a leaf, with more mature tissue 

 lying above it, the whole organ behaves as if this zone were a punctiim vegetatioiiis ; 

 the states of growth succeeding one another in the reverse order. Such a zone, in- 

 tercalated between mature portions of tissue may be called an Intercalary vegetative 

 zone. The growth of the internode or leaf may be termed basipetal, in contrast to 

 the acropetal development where the pimdum vegetationis lies at the apex of the 

 internode or leaf. 



According as the conditions of growth — temperature, the supply of water, 

 and light — are favourable, these phenomena proceed more or less rapidly and 

 uniformly. Every young cell formed at the pimctum vege/alw?n's grows and matures 

 more rapidly the more favourable these conditions are. But if the organs are 

 observed under the most constant possible conditions as they emerge from the 

 bud, it is seen that the growth of the organ both in length and thickness, de- 

 pendent on the gradual development of the cells, does not advance by any means 

 uniformly. The growing portion of a root, internode, or leaf does not lengthen 

 to an equal amount in equal consecutive intervals of time ; and the same is the 

 case with stems consisting of a number of internodes, and with each zone, however 

 small, of a growing organ. It is seen in fact that the growth of each part begins 

 at first slowly, becomes gradually more rapid, and finally attains a maximum of 



