826 PHYSIOLOGY OF THE ORGANS OF REPRODUCTION. 
to show the line of descent of the nutritive matter, is the fact, 
that if the cortical parts of the stems of a Potato plant be 
peeled off, the formation of tubers is prevented. It appears 
that the descending sap supplies the material for the formation 
of new wood in the fibro-vascular layers. The course of the 
sap is also lateral, for in the autumn starch grains are found 
in the medullary rays between the wedges of developed wood ; 
and where growth is going on, even an upward direction may 
be assumed. Herbert Spencer, however, argues that the re- 
trograde motion of the sap is through the same channels— 
chiefly, as he believes, the vessels of the newest wood—by which 
it passed up. He considers that this descent takes place in 
response to a demand for liquid by the stem and roots when 
transpiration from the leaves is at a standstill, as at night. As 
far as the leaf-petioles are concerned, the back current must be 
along much the same tissues as the upward flow ; but probably 
the liber-cells of the petiole are the main channel, and these are 
directly continuous with the inner bark of the stem. 
Spencer has also described and figured (Linn. Soc. Trans- 
actions, xxv.) cellular masses which he finds at the termination 
of the vascular system in the lower layer of parenchyma in 
many leaves, and which he considers to be undoubtedly absorbent 
organs by which the elaborated sap is abstracted from the leaves ; 
his conclusions, however, require confirmation. 
The opinions of observers vary much as to the offices of the 
different parts of plants ; for instance, Mulder considers that all 
nitrogenous matters are not only absorbed by the roots, but 
also assimilated by them at once, while carbon is fixed by the 
green parts; so that a constant interchange must take place 
between the leaves and roots. Other authors, again, believe 
that the leaves form all the organic substances. While Sachs 
says: ‘By the parenchyma of the fundamental tissue, which 
always has an acid reaction, are conveyed the carbo-hydrates 
and oils ; by the soft bast the mucilaginous albuminoids, which 
have an alkaline reaction.’ 
CHAPTER 3. 
PHYSIOLOGY OF THE ORGANS OF REPRODUCTION.” 
Havine now alluded to the special functions of the elementary 
structures, and of the organs of nutrition, and also to the general 
physiology or life of the whole plant, we proceed in the next 
place to treat of the functions of the Organs of Reproduction. 
1. Functions or Bracts AND FLoraL ENVELOPES.—One of 
the principal oftices performed by these organs is, to protect the 
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