CH. ri. 



COURSE OF THE SAP. 



77 



tlie stems to grow downward to the liglit, and the 

 roots upwards m the earth, some of the gemmules or 

 stems grew upward, and came out of the hole at the 

 then upper part of the inverted flower-pot. These 

 plants passed through eight inches and a half of 

 earth. On dividing them lengthwise I found that, in 

 those parts of the stem which grew in the air, the piths 

 and herbaceous envelopes were green, and in those 

 parts which grew in the earth they were white. The 

 piths ran tlie entire length of the roots as well as the 

 stems ; and where the necks of the plants divided the 

 stems from tlie roots, the two piths were continuous, 

 and of precisely equal size. It is the universal error 

 in physiology, to believe that roots have no piths. Let 

 any one divide a seedling horse-chestnut, and he will 

 convince himself. 



Each layer of bark is supposed to have its proper pith 

 or cellular ring outside it. The green cellular or ' herba- 

 ceous envelope ' under the outer cuticle is supposed to 

 be the pith of the outer layer of bark, and to be to the 

 layers of bark what the central pith is to the layers of 

 wood; and throughout its whole extent there is 

 probably a direct vascular communication between this 

 green external pith of the bark and the internal central 

 pith of the wood, by means of the medullary rays. 

 This green parenchymatous pith of the bark is in com- 

 munication with, and is in fact a continuation of, the 

 parenchymatous parts of the leaves (the spongy porous 

 parts, as distinguished from the woody fibrous parts), 



