CHAPTER III 



THE FXOLOGY OF STEMS 



Upright stems are largely concerned with the display of foliage 

 and with the conduction of food materials from roots to leaves and 

 of foods from the leaves to growing regions and to storage organs. 

 Prostrate and other horizontal stems often have the more special 

 function of vegetative reproduction. Stems may also serve as 

 storage organs. 



16. Foliage Display.— The efficiency of stems as organs of foliage 

 display depends largely upon their ability to elongate and to branch. 

 The direction of gro^^'th of both main shoots and branches is con- 

 trolled largely by gravity and light. The amount of elongation is 

 controlled partly by light and partly by water. 



One of the most efficient types of stem systems from this point 

 of view is that of excurrent trees well exemplified by pines and 

 spruces (Fig. 8). In such trees the shape is that of a cone, the 

 lowermost branches being the longest and the uppermost the short- 

 est. Leaves are borne only near the ends of these branches so that 

 the whole forms a sort of conical tent the walls of which are formed 

 of green leaves. Such a stem system makes possible the display of 

 an enormous number of leaves nearly all of which receive sufficient 

 light to enable them to carry on a maximum amount of food manu- 

 facture. 



There are many other types of stem systems which are nearly as 

 efficient as those of the excurrent trees. Herbaceous plants exhibit 

 a great variety of forms, many of them resembling those of trees. 

 In places where these herbaceous plants are not shaded by larger 

 plants they are fully as efficient as trees except that because of 

 their smaller size they cannot display so large a number of leaves. 



17. Vegetative Reproduction.— Vegetative reproduction in the 

 higher plants takes place almost entirely by means of stems. The 

 commonest and most efficient type of stem from this point of view 

 is the rhizome, or underground stem. In some kinds of plants, such 

 as most ferns, the rhizome constitutes the whole stem system of the 

 plant. In others there are annual aerial stems which bear the 

 leaves. Some rhizomes are practically unbranched while others 

 branch quite extensively. In any case it is characteristic of rhizomes 

 that they elongate each year and so advance into new territory. 



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