152 



BOTANY OF THE LIVING PLANT 



FIG. no. 



Transverse section of stem of Lamium, 

 showing projecting angles of collen- 

 chyma (dotted), opposite four larger 

 vascular strands : an arrangement equi- 

 valent to two crossed girders. ( x 14.) 



of the large number of the strands, and their slight irregularities ; and this is 

 common in herbaceous Dicotyledons. But in the case of Clematis only three 

 crossed girders enter into the construction, so that the method appears clearer : 



and still more so in Lamium, where there are 

 only two. There are other points of wide 

 application illustrated in these cases. The 

 stems are fluted, with projecting angles, and 

 one strap of mechanical tissue is seated in 

 each. This gives added strength on the 

 principle of the fluted column, the depth of 

 each girder being thereby increased. The 

 second point is that one of the stronger 

 vascular strands is opposite each of the 

 mechanically strengthened ridges, so that 

 the construction of the stem, as it grows 

 older and the vascular strands become 

 mechanically more effective, resembles 

 that of a number of peripheral girders 

 disposed in a ring. This method is seen 

 in the frames supporting large gasometers. 

 The central tissue may even be replaced by a cavity filled with air, 

 which gives added point to the comparison. The simpler construction of 

 the stem in Lamium may be compared mechanically with that of 

 a lattice signal post. In it four bands of metal occupy the four 

 angles, and are kept in place by a lattice- 

 work of thin straps, while the centre is 

 hollow, So in Lamium (Fig. no), the 

 projecting angles contain the chief 

 mechanical tissue The softer tissues 

 hold them in place, while there is a 

 central .pith-cavity It. is immaterial 

 exactly how the mechanical arrange- 

 ments are analysed in such stems as 

 these quoted. The point is that the 

 girder principle can be recognised in 

 them all, with strengthening strands 

 isolated and peripheral. But in Dico- 

 tyledons these arrangements are apt to 

 be lost sight of as the stems grow older, 

 owing to secondary thickening, and the 

 development of the vascular ring into 

 the central column of wood (p. 46, etc.). 

 The stem thus assumes at last the type of 

 the solid column. In large tropical trees 

 a further mechanical device is often seen 

 at the base. By unequal thickening broad 

 flanges are produced, which radiate outwards, and act in support like 

 the buttresses of a Gothic tower. The final result of such development might 

 be compared with the outline of the greatly widened base of the Eiffel Tower. 



FIG. in. 



Transverse section of the shaft oiScirpus 

 (Eleocharis) caespitosus, showing four large 

 girders, with smaller and less perfect girders 

 between them. Centrally is a large cavity. 

 The dotted areas indicate thin-walled 

 water-storage tissue. ( x 62.) 



