188 SCIENCE PROGRESS. 



became more strongly developed in proportion to those 

 parts above ground, and some species, which are annuals 

 in the plains, became perennials at the higher elevations. 

 These "transitory changes" answer, in general, to the 

 characteristic, hereditary peculiarities of Alpine plants. 

 Analogous instances might be quoted, particularly in the 

 cases of aquatic plants, and in still greater numbers. But 

 I would prefer to point out a few cases of the reaction of 

 tissues to the requirements of their surroundings. 



It is known that transpiration conditions affect the thick- 

 ness of the outer walls of the epidermal cells, together with 

 the cuticle, and even the development of the palisade paren- 

 chyma is influenced by similar conditions, and of the truth 

 of this Stahl's Licht-und Sc flatten- Blatter are an example. 



As a further fact of interest it may be mentioned that 

 the dimensions of the intercellular spaces increase in damp 

 air or in water. Hegler has recently described a particu- 

 larly striking instance of the effect of the environment on 

 tissue-development. If one exposes the growing parts of a 

 plant to a mechanical strain, the existing mechanical elements 

 increase in thickness and number, and even tissues, which 

 were lacking hitherto, may be formed. The hypocotyl of 

 the seedlings of Helianthus animus, which broke under a 

 weight of 1 60 grams, was able to bear 250 grams after two 

 days, and in a few clays 400 grams. In the leaf-stalks of 

 the Helleborus uiger there are normally no bast-fibres, but 

 on introducing a mechanical stretching or pulling strain they 

 appear and form effective strands round the phloem. The 

 faculty of forming these tissues was, therefore, latent in those 

 leaf-stalks, but it was only rendered evident in consequence 

 of the influence exercised by the mechanical strain. And 

 this leads to the discussion of the influences which, according 

 to Nageli, produce lasting and hereditary changes. This is 

 said to take place by means of forces which exert their 

 influence for very long periods, and for a long series of 

 generations, and in this way actually alter the construction 

 of the idioplasm. The extent of the direct influence cannot 

 be proved, but can only be deduced from general considera- 

 tions. According to Nageli evaporation acted as a stimulus, 



