[&J 



BOTANY OF THE LIVING PLANT 



f tlu- 1 irge Dumber of the strands, and their slight irregularities ; and this is 



lU a I ^cotyledons. But in the case of Clematis only three 

 , 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 

 principal 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 



l.oB. 



Fig. 117. 



Transverse section of stem of Lam ium , 

 showing projecting angles of collen- 

 rhyma (dotted), opposite four larger 



ular strands: an arrangement equi- disposed in a ring. This method is seen* 

 v.d.,n to two crossed girders. (xi<.) in the • frames suppor t in g 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. 117), 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 

 irelopment of the vascular ring into 



ttral column of wood (p. 58, etc.). Transverse section of 'the shaft of S C1 >/, MS 



I he Stem thus assumes at last the type of {Eleocharis) caespilosv.s, showing four large 



iv. i;j 1 T , . . girders, with smaller and less perfect girders 



the solid column. In large tropical trees between them. Centrally is a large cavity. 



a further mechanical device is often seen Thc dotted areas indicate thin-walled 



, . water-storage tissue. ( x 62.) 



at the base. By unequal thickening broad 



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



thr 1,ult: ,f a Gothic tower. The final result of such development might 



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



