47° 



LECTURE XXVIII. 



size does the diiferentiation of the vascular bundles take place, in the embryonic 

 tissue of the shoot-axis as well as the young leaf, as mentioned previously, so that the 

 vascular bundle curving out into the leaf represents the upper end of that which 

 descends in the shoot-axis. The differentiations of tissue are similar when a normal 

 growing-point of a shoot arises from a growing-point already existing ; that is, in the 

 branching of the shoot. The case is quite otherwise in the origin of new roots, 

 however, as already pointed out in the lectures on Organography. Whether the 

 roots arise from a mother-root or from a shoot they are always produced in the 

 interior of the tissue, so that the young root, when elongation begins at the base of 

 its growing-point, has to break through the external layers of tissue (cortex and 

 epidermis) of the mother-organ. Of course in this case also a complete continuity of 

 the homonymous layers of tissue comes to exist subsequently, but only subsequently, 



FIG. 308. — CauUrjfa crassi/olia. s the shoot-axis ; -v its growing-point ; i leaves ; tu roots. 



and the microsco'pic structure gives the impression that it is here so to speak a matter 

 of patchwork. 



For the subsequent form of the entire shoot it is important what portion of the 

 circumference the young leaf-rudiment occupies on the growing-point. In most 

 cases it is only a small part of the circumference, and obviously this is always the 

 case when two, three, or more leaves are put forth at one and the same transverse 

 zone, that is, when a whorl of leaves springs from the growing-point. In many 

 Monocotyledons (Grasses, Sedges, Aroids, Palms, &c.) and even in some families of 

 Dicotyledons, such as the Umbelliferae and Polygonacese, however, the whole circum^ 

 ference of a transverse zone of the growing-point is occupied by the rudiment of 

 a leaf. In such cases the base of the leaf appears on further development as a 

 sheath surrounding the shoot-axis, at the upper edge of which the true leaf, the 



