30 TQPOGRAPHY OF CHLOROPHYLL APPARATUS IN 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 growth of the stem, the distribution of the chlorophyll is changed 
and it becomes much reduced in amount. Ina 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 subepidermal chlorophyll band. The fate of the 
chlorophyll band was not learned. 
SAMBUCUS MEXICANA. 
Sambucus 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. Ina 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, 7. 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- 
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 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. 
