CONIFERALES 



259 



one year do not prosper under such intense civilization, and even de- 

 ciduous plants shed their leaves a little earlier in the autumn. 



Histology of the leaf. — The structure of the conifer leaf is xero- 

 phytic, in many cases exceedingly so. The needle of Pinus is typical 

 of the forms with needles on spur shoots (fig. 267, 268). There is a 

 thick layer of cutin, the epidermal 

 cells are small and thick walled and 

 the hypodermal cells, one to three 

 layers deep, are also thick walled. 

 The stomata are deeply sunken. Sur- 

 rounding the bundle area is a con- 

 spicuous endodermal sheath of large, 

 thick-walled cells, marked with the 

 prominent Casparian strips which 

 are so often found in endodermal 

 cells. Within the sheath there are 

 two bundles in Pinus laricio and one 

 in P. strohus; and, in both, there is 

 abundant transfusion tissue. 



Since many of the cells of the trans- 

 fusion tissue are more or less lignified 

 and have bordered pits, some be- 

 lieved that this tissue is a reduced 

 xylem portion of the bundle, an in- 

 terpretation which would make the 

 bundle mesarch. This feature is very 

 well shown in Sciadopitys verticellata, 

 where most of the cells beyond the 

 protoxylem could be interpreted as centripetal xylem (fig. 269, C). 



Although the leaf of Sciadopitys consists of two needles, fused 

 along the posterior margins, the internal structure shows no evidence 

 of a fusion, the tissue being quite uniform. In the concrescent leaves 

 of Thuja, and others of this type, the fusion is not so complete. The 

 leaves adhere strongly, but it is usually possible to recognize the 

 epidermis along the lines of contact. There are many large air spaces 

 in the leaf of Sciadopitys, and numerous sclerotic cells, with sharp, 

 projecting prongs, like those so conspicuous in the dicot, Nymphea. 



Fig. 263. — Thuja occidentalis: 

 end of branch showing appressed 

 leaves; natural size. See also fig. 266. 



