96 OTHER TYPES OF STEMS 



for lumber. The heart wood is composed of dead cells and 

 while the most serviceable for building purposes, it is of little 

 use to the tree as we often see a vigorous tree whose heart wood 

 has been largely destroyed by decay. We ought not to leave this 

 vascular system of the plant without again emphasizing the per- 

 fection of it. First there is the completeness of the system, ex- 

 tending from the root to all parts of the leaf. As soon as the root 

 hairs begin to form and function there is already at that point the 

 beginnings of one of the terminals of the system while the other 

 terminal is also appearing in the leaves while they are forming and 

 still concealed in the buds. Then the elongated cells, those of 

 the xylem being in addition largely without living cell contents, 

 ensures a rapid transport of material. The weak feature of this 

 system is the terminals for here the materials must not only pass 

 through the numerous walls of the short cells but also through 

 the still more impermeable membranes of the living matter of 

 these cells. But once the material is landed in the elongated' 

 cells, then follows a rapid transport. Therefore it is the develop- 

 ment of this system that makes possible the stature of plants. 

 Without it, plants must remain small and in close reach of their 

 crude supply. 



41. Other Types of Stems. — The cone bearing trees, such as 

 the pines and spruces, have essentially the same arrangement 

 of tissues and mode of growth as noted above in the dicotyledons. 

 The xylem, however, consists entirely of tracheids (Fig. 59) 

 with the exception of a few small spiral vessels that are formed 

 as the first cells of the vascular bundles. 



The monocotyledons, plants distinguished usually by their 

 parallel veined leaves and single seed leaf, like the palms, lilies, 

 grasses, etc., are characterized by stems that do not increase 

 materially in diameter. Growth is largely confined to the top 

 of the stem and consequently it can only elongate, forming a 

 very regular, columnar trunk. The reason of this is apparent 

 when cross sections of such stems are examined (Fig. 46, D). 

 The vascular bundles are more or less scattered throughout the 

 stem. The cause of this arrangement is seen in the seedling 

 stage of the plant. Here we see the vascular bundles arising 



