48 THE PHYSIOLOGY OF PLANTS 



pensable, and obligatory parasites (mistletoe), contrasted with 

 occasional absorbers of humus (forest trees) and occasional 

 parasites (cow-wheat, yellow rattle). As a similar and equally 

 important special method of nutrition in the vegetable king- 

 dom, we must, in our present state of knowledge, consider the 

 root tubercles of leguminous plants. 



If we take up carefully peas, lupins, beans, robinias or true 

 acacias, and other Leguminos^e, which have been grown in 

 sand, and rinse the sand from their roots, we find the main 

 root in some cases, in others the lateral rootlets, bearing 

 curious fleshy tubercles, which are sometimes of characteristic 

 shape for different plants. These generally have the appear- 

 ance of a rudimentary tubercular rootlet, and in the first 

 stages of their development appear to the naked eye very 

 much like a lateral root breaking through the cortical tissue. 

 But in reality their origin is a very different one. These 

 tubercles arise by active cell division of the inner layers of 

 the cortex, while the lateral roots arise from the pericambium. 

 The pericambial layer, however, only takes part later on in 

 the formation of the tubercles. At the periphery of the 

 tubercles there will be observed a layer of cambiform cells, 

 which produce towards the outside large thick-walled cortical 

 cells, and towards the inside a smaller-celled tissue, and also a 

 ring of fibro-vascular bundles, which join on to the bundles of 

 the root. As the tubercles have for some time a meriste- 

 matic apex, by which they continue to grow, their similarity 

 to a short succulent rootlet is still further increased. But, of 

 course, they are not provided with a root cap. In the older 

 tissues behind the meristematic apex there gradually accumu- 

 late large quantities of oval or rod-shaped, undivided or forked 

 bodies, which are capable of moving about in water for many 

 days. These small bodies can be shown to consist of albu- 

 minous substances, and therefore characterise the tubercles as 

 storage tissues for nitrogenous material (albumens), which 

 are gradually used up by the plant during the ripening of 

 the seeds. 



The tubercles appear in greater numbers the poorer the soil 

 is in humus and soluble nitrogenous substances, especially 

 nitrates. If the soil is sterilised by heat and soluble nitrates 



