IX Leaves 177 



the interpretation of this, showing that in a focus of intense 

 Hght (from which, of course, all possibility of the destructive 

 action of heat has been removed by stopping this off by a 

 light transparent, yet heat-opaque alum screen) chlorophyll 

 is at once destroyed and no assimilation can therefore go 

 on. That different plants have different optimum light- 

 quantities for assimilation, just as they have different optimum 

 heat-quantities or temperatures for germination or growth, is 

 obvious enough ; but nature does not constantly supply 

 these ; how beautiful this self-regulating mechanism, by which 

 the chlorophyll grains can utilise just their right amount 

 of sunlight, making the most of it when it is scanty, and 

 escaping its dangers when excessive ! Nay more, may we 

 not thus explain the peculiar shape and position of the 

 palisade cells themselves ? Is it not to give the chlorophyll 

 granules room to assume the most favourable position that 

 they have elongated perpendicularly to the leaf surface ? 

 In the same way, since the cells of the deeper-lying, spongy 

 parenchyma are shaded by the layers above, and need all 

 the light they can get for their chlorophyll grains, must not 

 they expand horizontally ? And hence we should be able 

 to explain why it is that in shade-loving plants palisade 

 parenchyma may even be absent, and why in shade-grown 

 leaves the spongy parenchyma is often observ^ed to be more 

 abundantly developed. 



The explanation is beautiful, and seems a satisfactory 

 one ; a few years ago botanists were wont to teach it with- 

 out reservations. The critic, however, soon came in the 

 person of one of our most thoughtful and physiologically- 

 minded of vegetable histologists, Professor Haberlandt of 

 Graz, to whom, with his master Schwendener, belongs 

 especially the credit of attaching physiological interpreta- 

 tions to what were formerly too often mere microscopic 

 curiosities. He first verifies and establishes Stahl's 



N 



