MODIFIED FORMS OF SHOOT 325 



primary xylem, the pitted vessels following on the 

 spiral tracheids of the protoxylem. A band of tissue 

 between the xylem and phloem I'emains undifferentiated, 

 and this is the seat of the cambium or secondary vascular 

 meristem in stems which undergo secondary thickening. 



The tissues of the leaf keep pace in differentiation 

 with those of the stem, the leaf blade expanding, while 

 the epidermis, veins and mesophyll acquire their adult 

 characters. 



Modified Forms o! Shoot. — While the typical aerial 

 shoot stands erect in the air, other forms do not develop 

 the mechanical tissues required to maintain the erect 

 position. Some of these trail on the ground, others 

 twine round any support (such as the erect stem of an 

 upright plant, the side of the stem away from the support 

 growing faster than the side touching it). Other shoots, 

 as we have already seen (Chapter XV), grow below the 

 surface of the soil and do not bear foliage leaves, but 

 only scale leaves. Others, again, grow very little in 

 length but increase in diameter (" rootstocks," corms, 

 tubers). All these types of shoot have the same essential 

 plan of construction as the typical erect aerial stem, 

 but they differ very widely in details. Thus the cortex 

 may be relatively very broad, and it may contain 

 vascular bundles in addition to those of the cylinder 

 In some cases the vascular cylinder early loses its 

 identity altogether, so that the bundles are scattered 

 through the stem. Chlorophyll is not developed in 

 subterranean shoots, nor do they, as a rule, bear stomata. 

 Adventitious roots are practically always produced on 

 subterranean shoots, and very often also on creeping 

 shoots, especially at the nodes, as well as from the 

 bases of erect aerial shoots below the surface of 

 the soil. 



