96 LECTURE VI. 



inherent in the absorbent cells themselves. It has doubtless 

 been induced in plants by the daily variations of external 

 conditions, perhaps more especially of illumination, which 

 are involved in the alternation of day and night ; but it has 

 become so much a part of the nature of plants that it is 

 exhibited even when the conditions which originally induced 

 it are not present, and it is transmitted from generation to 

 generation. We shall find hereafter that many of the pheno- 

 mena of plant-life are periodic, and their periodicity has 

 doubtless been induced in like manner. 



But the vessels of the wood of plants do not always con- 

 tain watery fluid. Hales observed that whereas a Vine will 

 bleed freely if its stem be cut in the month of April, no bleed- 

 ing will take place if it be cut in July. And yet, since in July 

 the foliage of the plant is fully developed, and it is losing 

 considerable quantities of water by transpiration, it is evident, 

 as Hales did not fail to point out, that a current of water must 

 be passing through the stem from the roots to the leaves. 

 This current we will term the transpiration-current. What, 

 then, are the channels by which this current travels, if it 

 is not by the cavities of the vessels ? An answer to this 

 question is suggested by the structure of those vascular 

 plants submerged water-plants which lose no water by 

 transpiration, and in which, therefore, the current in question 

 does not exist. An examination of a transverse section of the 

 stem of one of these plants sheVs that the wood is but feebly 

 developed, and that the walls of its constituent cells are but 

 slightly lignified if at all. Inasmuch, then, as the wood of 

 these plants, which do not transpire, differs so much in struc- 

 ture from that of those which do transpire, it must be 

 inferred that the wood is of some importance in connexion 

 with this function. The correctness of this inference can be 

 proved experimentally. If a ring of cortical tissue, extending 

 inwards as far as the cambium, be removed from the stem of a 

 dicotyledonous plant, it will be found that the leaves, which 

 are borne on branches arising from the stem above the level 

 at which the ring has been removed, will not exhibit any signs 

 of withering, and Knight found that this was also the case 



