i88 



STORAGE OF FOOD AND WATER 



sometimes all of the pith; and the phloem parenchyma and 

 thin-walled parenchyma of cortex and pericycle. 



The storage tissues in seeds make up their greatest bulk, 

 and, unless the cotyledons are employed in storage, the embryo 

 is only an insignificant part of the seed in size. When cotyle- 

 dons that have been used for food storage are coming above 

 the ground and turning green their mesophyll cells are deliv- 

 ering up the stored food so that the cotyledons gradually become 

 thin, and the mesophyll, through the secretion of chlorophyll 



by its leucoplasts (which then become chloro- 

 plasts) enters upon its new function of photo- 

 synthesis. 



The medullary rays are one of the most 

 important of the storage tissues. Although 

 the vast majority of the rays are very small, 

 ^tending only a few cells vertically, and 

 being one cell, or at most a few cells, wide 

 tangentially, while they run to various dis- 

 tances toward the pith, they are yet so 

 numerous that their bulk would sum up 

 approximately lo per cent, of that of all 

 tissues of the wood. The smallest medullary 

 rays are passed unnoticed by the naked eye, 

 but these compose most of the medullary ray 

 tissue. In a tangential section of wood one 

 centimeter square where the naked eye dis- 

 tinguishes no medullary rays, the microscope 

 reveals nearly three thousand, or there would 

 be about thirty rays in a single square millimeter. These 

 rays are thickly scattered amongst the other tissues and with 

 great frequency come into contact with the tracheal tissues 

 and wood parenchyma. The rule is that every ray touches one 

 to many tracheal tubes, and when in longitudinal sections of 

 wood a tracheal tube is examined along its length it is found that 

 it comes into contact with about thirty rays for every centimeter 

 (Fig. 102). These rays not only extend into the wood but also 



Fig. 102. — Camera- 

 lucida, low-power 

 drawing of tangential 

 section of wood of 

 Liriodendron tulipifera, 

 showing frequency of 

 contact of medullary 

 rays with a tracheal 

 tube. /, medullary 

 ray; g, tracheal tube. 



