8l6 MECHANICS OF GROWTH. 



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 commonly found at the upper end of the internode (as in Phaseolus)^ and less 

 frequently at the lower end, and this usually when it is enveloped by closely appressed 

 leaf-sheaths or when it is in a bulb, as e. g. in Equisetaceae (especially E. hyemale), 

 Umbelliferae, the bulbous Liliaceae, the haulms of Grasses, &c. If the internodes are 

 not sharply defined, 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 in stems 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 Papilionacese or the incised leaves 

 of Araliaceae. But very often the activity of the puncium vegeiationis of the leaves 

 lasts for only a short time and its tissue completes its growth 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 Liliacese 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 puncium vegeiationis ; the states of growth succeeding one another in the reverse 

 order. Such a zone, intercalated between mature portions of tissue, may be called 

 an Iniercaiary Vegeiative Zone. The growth of the internode or leaf may be termed 

 basipetal, in contrast to the acropetal development where the puncium vegeiaiionis 

 lies at the apex of the internode or leaf. 



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

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

 uniformly. Every young cell formed at the puncium vegeiaiionis 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 their growth, both in length and thickness, dependent 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 con- 

 sisting 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 rapidity, after 

 which the growth becomes again slower, and finally ceases when the organ is 

 fully mature. 



If successive equal intervals of time are represented by T^, T^... T„, and the 

 increments during these intervals by I^, I2, ... I„, then it may be stated as a general 

 rule that — 



