30 TQPOGRAPHV OF CHLOROPHYLL APPARATUS IX DESERT PLANTS. 



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 usually the 

 case with g-rowth of the stem, the distribution of the chlorophyll is chang-ed 

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

 and 90 cm. from the ti]) 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 foimd onh- in the subepidermal chlorophyll band. The fate of the 

 chlorophyll band was not learned. 



SAMP.UCUS MEXICANA. .. 



Samd7(n/s 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 topogfraphy of the chlorophyll apparatus presents no uniisual char- 

 acters. In the young-est portions of the branch, 2.5 mm. in diameter and 

 0.5 cm. from the tip, all of the g-round-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 

 spongry 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 youngrest portions of the stem are frequently green, /. e., there is 

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

 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- 

 cambium 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 transjiiration, either the 

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

 or cork is q:iven the chlorcnchvma during its entire existence. 



