INTRODUCTION 



69 



in Mnium undulatum (Figs. 28 and 29). A beautiful example of the 

 transition first mentioned is afforded by the growth of Hypnum splendens. 

 This moss, growing in the shade of woods, possesses (see Fig. 27) 

 distichously-branched shoots which are expanded at right angles to the 

 incident light and resemble very closely pinnate leaves. Each of these 

 plagiotropous shoot-systems bears small leaves and is only capable of 

 vegetation during one vegetative period. At the beginning of the next 

 period there develops near its base a strong orthotropous lateral shoot 

 which remains unbranched ; it soon becomes plagiotropous, branches 

 distichously, and spreads out in a plane at right angles to rays of incident 

 light. As the old dying-off generations of shoots remain for some 

 little time there is developed a tiered construction which prevents the 



O~-f<\ ll ,iJ' .4 , _---. 



FIG. 28. Mnium undulatum. Vegetative shoot. It is at first orthotropous and afterwards plagiotropous. 

 Natural size. 



FIG. 29. Mnium undulatum. Orthotropous shoot ending in a group of antheridia girt by a rosette of leaves; 

 below this three shoots, plagiotropous from the outset, arise on the orthotropous chief shoot. Natural size. 



new shoot-generation from being buried in the detritus of the soil of 

 the wood. 



To a certain extent the behaviour of the pine is analogous ; it 

 possesses orthotropous chief shoots and plagiotropous lateral shoots, 

 but the outgrowing tips of the new lateral shoots in spring are at first 

 orthotropous and it is only later, evidently through correlation and 

 under the influence of the chief shoot, that they become plagiotropous. 



Many trees possess, at least in their later years of life, exclusively 

 plagiotropous shoots, although the plant exhibits the configuration of 

 an orthotropous chief stem ; this is however produced by the concatenation 



