64 STRUCTURE OF THE STEM 



77. Structural units of the dicotyledonous stem. The stu- 

 dent should already, from his own examinations, have learned 

 a good deal about the kinds of cells and cell aggregates which 

 compose the stem. The preceding figures (Figs. 56-60) will serve 

 to illustrate the most important of these, and Figs. 61-64 show 

 some of them more in detail. 



78. Parenchyma, prosenchyma, and collenchyma. A mass 

 of similar cooperating cells is called a tissue. 1 Two of the prin- 

 cipal classes not previously mentioned which occur in the stem 

 are paremhymatous tissue and prosencliymatous tissue. Paren- 

 chyma is well illustrated by the green layer of the bark, by 

 wood parenchyma, and by pith. Its cells are usually somewhat 

 roundish or cubical, at any rate not many times longer than 

 wide, and at first rather full of protoplasm. Their walls are 

 not generally very thick. Prosenchyma, illustrated by hard 



bast and masses of wood cells, consists of 

 thick-walled cells many times longer than 

 wide, containing little protoplasm and often 

 having little or no cell cavity. 



As a rule the stems of the most highly 

 developed plants owe their toughness and 

 their stiffness mainly to prosenchymatous 

 tissue. In some stems, particularly the 

 fleshy ones, the stiffness is, however, largely 

 FIG. 65. Collenchyma- due to collenchyma, a kind of parenchyma 

 tous and other tissue in which the cellg are thickened or reen- 

 from stem of balsam . . _ r 



(Impatient) forced at their angles, as shown in Fig. 65. 



e, epidermis; c, coiien- 79. The early history of the stem. In 

 chyma; i, intercellular ^he earliest stages of the growth of the 



spaces between large . . . . . ,, , 



parenchyma cells. stem it consists entirely of thin-walled 

 After strasburger an( j ra pidly dividing cells. Soon, however, 



the various kinds of tissue which are found in the full-grown 



stem begin to appear. 



1 See Strasburger, Noll, Schenck, and Schimper, Text-Book of Botany, 

 pp. 92-95, 2d ed., London, 1903. 



