40 



LECTURE IV. 



Stand in quite a different relation towards the shoot-axis : the epidermis of the latter 

 passes over to the leaf without visible interruption, and the cortical tissue of both 

 is also in complete continuity, and, apart from a few cases, the vascular bundles 

 of the stem or shoot-axis are really nothing more than the lower ends of the 

 vascular bundles which curve out above into the leaves, and produce in these the 

 so-called venation. In its original condition, indeed, the shoot-axis has generally 

 no surface of its own at all, since the leaves come forth so closely one upon an- 

 other at the growing point, that no free surface of the shoot-axis remains. If the 

 growth in length of the latter is very slight, even the completely formed shoot- 

 axis has no free surface ; it is quite covered with leaves, as in our wood-fern, 

 Aspidium Filix-mas, and in such stems of phanerogams as bear their leaves in 

 so-called radical rosettes, and in many other cases. But even where a more vigorous' 

 growth in length of the shoot occurs, the bases of the leaves may grow with it, in 

 such a manner that the whole surface of the shoot-axis is nevertheless clothed with 



FIG. 30.— Upper end of a shoot-axis of 

 Hippuris Tiutffaris, after removal of tile 

 surrounding older leaves. Above is the 

 naked growing point, from which arise the 

 young leaves visible lower down (mag- 

 nified). 



Fig. 31.— Longitudinal section through the apex 

 of an erect shoot of Hzpfiuris vulgaris, s apex of 

 shoot : b, b, b leaves (in whorls) ; k, k, their axillary 

 buds, which become flowers ; ff, g primary vessels. 

 The dark parts indicate the cortex, with its intercel- 

 lular spaces. 



leaf-substance, as occurs especially with very small leaves, e.g. in the SelagimM 

 and many Cupressinea, such as Thuja. In the typical development of a shoot, 

 however, the growth of the shoot-axis proceeds in such a manner that the bases 

 of the leaves, at first closely crowded, become separated, since between each two 

 superimposed leaves of the growing point, a piece of the shoot-axis is as it were 

 mtercalated, and then attains a more or less considerable length. Such a portion 

 of the axis is then called an internode, or inter-foliar part of the shoot-axis. 

 Only among the vascular cryptogams are single cases known {Salviniacece) where, 

 immediately on the development of the leaves at the growing point, such inter- 

 fohar parts of the axis with free surfaces are present. One of the most striking 

 phenomena in shoots, viz. the position of the leaves on their axis, can be in part 

 explained causally from the fact that the youngest leaves arise at the growing point 

 as said, so close above and by tlje side of one another, that the whole surface of 

 the shoot-axis is covered by them, so that with the growth of the leaves mutual 

 pressure must necessarily occur. 



