78 PRACTICAL BOTANY 



tubers, or bulbs, before the trees under which they grow are 

 in full leaf, so as to shut out the abundant light necessary 

 for photosynthesis. 



Fleshy leaves often contain much stored food, as in the 

 familiar century plant (Figs. 62 and 63). This receives its 

 name from the commonly received idea that it must store food 

 for a century before it can blossom. In hot climates, however, 

 such as that of Arizona, near Tucson, it flowers at the age of 

 fifteen years or but little more. By the end of the flowering 

 season the leaves have lost more than 90 per cent of their 

 weight, which has been expended in producing the immense 

 flowering shoot. This may reach a height of over 33 feet and 

 a weight of some 500 pounds. Its average growth in height 

 during the month of most rapid elongation has been found to 

 be about five and one half inches a day. Not only the plant 

 food, but also nearly all of the water for this rapid growth is 

 furnished by the leaves. 



70. Food for reserve stores brought from elsewhere. In all 

 plants of high organization the reserve food is carried from 

 the cells in which it was manufactured mto other cells. In 

 plants with fleshy leaves, like the houseleek, the century plant, 

 the common purslane, and many others, the greater part of the 

 stored starch and other nutritive materials has only been 

 carried from the outer portions of the leaf, where photosyn- 

 thesis and other manufacturing processes go on, into the leaf 

 interior. The distance traversed may be only a small fraction 

 of an inch. But in case much of the food is stored in under- 

 ground parts of the plant it may have been carried for long 

 distances, in large trees even much more than a hundred feet. 



71. Form in which plant food is carried. As is suggested 

 in Sect. 17, the first visible product of photosynthesis in most 

 plants is starch. This is deposited m or about the substance 

 of the chloroplasts, during theu- exposure to daylight, in the 

 form of very minute grains. In the course of the. night these 

 disappear, so that testing a leaf with iodine ^ shortly before 



^ This turns starch grains blue or almost black. 



