PINUS AUSTRIACA, OR P. LARICIO. 141 



disappear, leaving the vessels as continuous tubes. In 

 tracheids the end walls do not disappear. As the tra- 

 cheids in the pines and their kin develop, the lateral 

 walls, originally thin and plain, thicken irregularly, a 

 part of the thickening on each side of the primary wall 

 growing away from it to form the "border" of the small 

 spot that remains thin, the whole constituting the "bor- 

 dered pit." Usually the primary wall remains as a 

 membrane separating the two cells. Should it be de- 

 stroyed, there would be free communication between the 

 contiguous cells. 



Outside the xylem region is the cambium tissue, com- 

 posed of a few layers of thin-walled active cells from 

 which during the growing season new cells are constantly 

 formed by division. In the phloem just outside the 

 cambium the leading tissue- element is the sieve-cells. 

 These are elongated cells, the side and end walls of which 

 have become perforated, thereby forming " sieve- plates," 

 through which food may pass. The "sieve-cells" are 

 used largely as transporting and temporary storage 

 regions for foods. Outside the phloem region the layers 

 of living and dead cortex are formed. 



The scale-leaves are entirely protective, while the 

 foliage work is almost exclusively done by the needle- 

 leaves. The epidermis and sclerenchyma of the needle- 

 leaves are especially thick- walled and serve to give rigidity 

 to the leaves, and to protect the chlorophyll-bearing 

 tissues against rapid changes in temperature and too 

 great loss of water. The sunken position of the guard- 

 cells secures them against stoppage which would hinder 

 the entrance of necessary gases. 



