290 The Plamt World. 



the "cement zone," where such deciduous trees as the pear, 

 apple, peach, apricot, plum and fig were but slightly damaged 

 in comparison with the citrus trees. On these latter, one not- 

 iced, first of all, a great deficiency of foliage. This is dense in a 

 healthy tree, but, as some of the orange growers phrased it, 

 "one could see right through" the dust-covered trees, many of 

 the leaves falling prematurely. To a marked extent many 

 leaves had failed to develop to their full size, so that there were 

 numerous undersized ones, and these often stood erect, in place of 

 assuming a nearly horizontal position as do healthy leaves. 

 Lemon trees exhibited a twiggy appearance, many shoots being 

 bare except for a leaf or two at the tip. There was a consider- 

 able difference of resistance shown by different varieties of the 

 orange, seedling trees suffering the least and Mediterranean 

 sweets the most. But with them all, fruit production, the fea- 

 ture most important to the grower, had greatly decreased, the 

 trees yielding now only about one-fourth as much as in pre- 

 cement years. The fruit has also deteriorated, a larger propor- 

 tion of it going into the lower grades and the "culls" or unmar- 

 ketable fruit. 



Many of the most experienced orange growers testified that 

 the situation and the soil were ideal for citrus culture, as was 

 evidenced by the former healthfulness and productivity of the 

 orchards. It was further shown that the orchards had been 

 cultivated, fertilized and irrigated in the most thorough and 

 approved manner. 



Such being the facts in the case, what was the cause of the 

 miserable condition of the trees? To the plant physiologist the 

 answer was evident and easy. With a rich soil and abundant 

 water the trees were plainly starving because the heavily shaded 

 leaves were prevented from manufacturing sufficient carbohy- 

 drates to maintain a healthy growth. 



On the removal of the cement scale the leaves were seen to 

 be in a more or less chlorotic condition, showing a deterioration 

 of the chlorophyll to etiolin. Dr. Peirce's careful microscopic 

 examinations demonstrated that a leaf whose surface was cement 

 coated produced but one-fourth as many starch granules as an 

 equal area of clean leaf. Thus, deprived of the normal number 

 of its leaves, with those that remained reduced in area, and with 

 three out of four of its chloroplasts inhibited in their activity, 



