40 



BRITISH FLOWERING PLANTS 



pith 



■parenchyma 



conditcting 

 woody Jibres 



Fig. 27. — Diagram of young cUcotyledonoiis stem, 

 showing approximation of the distinct bundle.?. 



mainly in two directions at right angles to one another, 

 and the stem would require additional strength in two 

 directions — in fact, the double girder (Fig. 24). This 

 being so, for plants with opposite decussate leaves 



quadrangular stems 

 would be most suit- 

 able. Three excep- 

 tions, however, at 

 least must be made. 

 In trailing and 

 water plants 

 strength is less re- 

 quired. The first 

 are supported by 

 the ground, and 

 little exposed to 

 the wind. The second are supported by the Avater in 

 which they live. 



As regards trees, the twigs grow in all directions, 

 and the tree therefore, as a whole, has to meet strains 

 in all directions. From this point of view it is interest- 

 ing that there are some cases of shrubby plants — such 

 as the Box — where the young shoots are quadrangular, 

 but gradually become round. We should expect, there- 

 fore, to find quadrangular stems mainly in erect 

 herbaceous plants with opposite leaves. Now let us 

 see how far the facts bear this out. Do quadrangular 

 stems and opposite leaves go together ? Asa matter 

 of fact all our English flowering plants with square 

 stems have opposite leaves. It is not so invariably the 

 case that plants with opposite leaves have square stems, 

 but for herbaceous plants it is the rule, and the ex- 

 ceptions are susceptible of explanation. Since, then, we 

 find no cases of quadrangular stems without opposite 

 leaves, and that most species which have opposite leaA^es 

 have quadrangular stems, it is a fair inference that the 

 form of the stem is connected with the arrangement of 

 the leaves. The pentagonal form of the Bramble stem 

 has been already referred to (p. 13). 



