424 



LECTURE XXVI. 



In those of the Grasses and Liliacece (e. g. Maize, Fritillaria, &c.) elongation is 

 completed about 8-10 mm. behind the growing-point; but even 50-60 mm. further 

 back the bordered pits of the large vessels are found to be only just fully developed, 

 and similarly with other tissue-structures. The internal development and strengthen- 

 ing of the tissues, which continue for some time after the completion of elongation, 

 are also plainly to be recognised from the exterior of the organs. Leaves and 

 shoot-axes which have just finished elongating are delicate, watery, easily torn, and 

 soon droop ; whereas they subsequently become much stronger, richer in substance, 

 and more solid. 



It would be superfluous to formulate the characteristics of this third stage of 

 growth, since all that has been said in the lectures on Organography and on the 

 theory of the tissues, with regard to the external form and internal structure of the 

 organs of the plant, concerns this last developmental stage. Only one point further 



need be mentioned here. With the exist- 

 ^ ence of the cambium in the shoot-axes 



and older roots of the Conifers and 

 woody Dicotyledons, of course a new 

 process of growth is ushered in, and the 

 cambium corresponds in many respects 

 to the embryonic tissue of the growing- 

 point. It is distinguished from this latter, 

 however, in that, normally, it only produces 

 new masses of tissue, but not new organs : 

 nevertheless the latter process also may 

 take place under certain circumstances. 

 In the case of cut-off woody stems, for 

 example, the cambium swells, grows out, 

 and forms a so-called callus — i.e. a cushion 

 consisting of soft tissue — in which growing- 

 points for the development of new shoots 

 are developed. 



The three phases of growth here dis- 

 tinguished usually pass into one another without interruption, so that no definite 

 line can be drawn between the embryonic condition and that of elongation, or 

 between the latter and that in which the internal development is being completed. 

 The relation between them is very simple in the roots; whereas the most various 

 complications may occur in shoots, according to the nature of the plant in each case. 

 The growing-point of a shoot is only apparently merely the end of the shoot- 

 axis, as it is generally held to be. The manner in which the leaves and lateral 

 shoots are developed from the growing-point of a shoot shows, on the contrary, that 

 the latter is not merely the termination of the shoot-axis, but is the embryonic be- 

 ginning of the entire shoot, which, in its turn, consists of shoot-axis and leaves. 



Just as new growing-points of shoots can arise from growing-points of shoots, so 

 also on a young leaf, so long as it still consists of embryonic tissue, a large number 

 of secondary growing-points may arise, which may then produce tertiary growing- 

 points, and even those of a higher order, as shown in Fig. 261. Hence the sub- 



FIG. 260.— Longitudinal section through the apex of an 

 erect shoot of Hippuris ■vulgaris, s apex of stem ; bdd the 

 leaves (in whorls); ii axillary buds of the latter and which 

 all become flowers ; £-—g the first vessels. The dark parts of the 

 tissue indicate the internal cortex with its intercellular spaces. 



