ix Leaves 



177 



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

 light (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 observed 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 



