30 TQPOGRAPHY OF CHLOROPHYLL APPARATUS IX DESERT PLAXTS. 



The chlorophyll occurs in the parenchyma, which is immediately within 

 the many-celled protective layer but not in a well-defined band, in the 

 medullary rays of cortex and of wood, and in the pith. As is tisiially the 

 case with growth of the stem, the distribution of the chlorophyll is changed 

 and it becomes much reduced in amount. In a branch 8.5 mm. in diameter 

 and 90 cm. from the tip the chlorophyll was confined to the outer portions 

 of the medullary rays and to the cortex; and in a branch 1.3 cm. in diameter 

 and 150 cm. from the tip it had left the woody cylinder entirely and was to 

 be found only in the subepi dermal chlorophyll band. The fate of the 

 chlorophyll band was not learned. 



SAMBUCUS MEXICANA. 



Sambncus occurs as scattered individuals by roadsides on the river-bottoms. 

 It forms a small tree from 5 to 8 m. high, with a main stem 15 to 20 cm. 

 in diameter. The tree which was selected for study is growing by the 

 Hospital Road east of the Laboratory domain. It is about 6 m. high 

 and has a polled appearance, as if most of the shoots were second growth. 



The topography of the chlorophyll apparatus presents no unusual char- 

 acters. In the youngest portions of the branch, 2.5 mm. in diameter and 

 0.5 cm. from the tip, all of the ground-tissue is chlorophyll-bearing. That 

 is to say, chlorophyll is to be found from the epidermis to the center of the 

 extensive pith and in all tissues except those already differentiated. There 

 appears to be no distinct cortical band. The chlorenchyma is made up of 

 spongy tissue, which for the most part has very thin walls and prominent 

 intercellular spaces. No change in the distribution of the chlorophyll is 

 to be noted until the stem is about 4.5 mm. in diameter, when none may 

 be found in the pith and but little in the inner portion of the cortex outside 

 of the medullary rays. It occurs in the medullary rays of the wood. 



When chlorophyll wholly left the stem was not determined. In a branch 

 8 mm. in diameter and 25 cm. from the tip, chlorophyll was found in the 

 parenchyma of the primary cortex immediately within the mechanical 

 tissue and nowhere else. In another branch 1.6 cm. in diameter and 

 at a point 1.23 m. from the tip the distribution was found to be quite the 

 same. 



The youngest portions of the stem are frequently green, /. e. , there is 

 no protective covering for the chlorophyll. The second node is often purple 

 from a red-blue pigment in the subepidermal cells. In the next older node 

 there may in addition be a heavy covering of trichomes. Also, the cork- 

 cambiimi is laid down early in the development of the branch and takes its 

 origin in the subepidermal cells, so that it happens that some sort of pro- 

 tection against excessive illumination or excessive transpiration, either the 

 leafy covering of the stem, pigmented cells, a hairy coating of the epidermis, 

 or cork is given the chlorenchyma during its entire existence. 



