IRRITABILITY. 393 



Before we leave this subject it may be incidentally men- 

 tioned that organs other than internodes become excessively 

 elongated when grown in darkness. Excessive elongation 

 is exhibited by the stalks, which belong to the category of 

 shoots in the more extended sense in which Sachs uses the 

 term, which bear the fructification of various Fungi. Brefeld 

 mentions, for instance, that whereas the subaerial hypha which 

 bears the sporangium of Pilobolus microsporus usually attains 

 a length of about half an inch, when grown in darkness it 

 attains a length of from eight to ten inches, and under these 

 circumstances no sporangium is produced : and again, that 

 the stipe of Coprinus stercorarius, which is usually only an inch 

 or so long, may attain in darkness a length of two feet, the 

 pileus remaining rudimentary. 



From the facts with which we have just become acquainted 

 we learn that the absence of light is, as a rule, favourable 

 to the growth in length of shoots, and from this purely negative 

 evidence we may conclude that light tends to retard growth 

 in length. But there is abundant positive evidence forthcoming 

 to establish this point. Sachs has observed, for instance, that 

 if a potato be allowed to sprout, exposed as fully as possible 

 to light, the shoots attain only a very inconsiderable length. 

 In one case shoots thus exposed for nearly eight weeks were 

 only a little more than one centimetre long, whereas etiolated 

 shoots of the same age were 15 20 centimetres long. The 

 shoots of the Potato are normally developed under ground, so 

 that they pass through the first stage of their growth in more 

 or less complete darkness ; they appear to be adapted for 

 growth in darkness during this stage, so that when they are 

 exposed to light the effect of its retarding influence is shewn 

 in the conspicuous manner described above. 



The retarding influence of light upon growth in length has 

 also been determined directly by comparative measurements. 

 Strehl found the relative elongation during a period of eighteen 

 days, of the roots and hypocotyls of seedlings of Liipinus albus, 

 some of which were grown in darkness and an equal number 

 (34) under the normal alternation of day and night, to be as 

 follows : 



