CHAP, ii.) VEGETAL NUTRITION. 107 



tissue, forasmuch as it came neither from the leaves nor from 

 the roots. It had been already prepared, and must have 

 reached the periphery by means of the transversal medullary 

 radii. 



It is probably because the monocotyledonous plants have none 

 of these transversal medullary radii that there is not in them 

 any flow of sap between the cortical system and the central 

 system, and consequently no growth at the point of junction of 

 these two systems. 1 



In sum, there is no true -circulation of sap, forasmuch as the 

 course of the nourishing liquid is at the mercy of a number of 

 accidents. There are only two principal sources of the sap : 

 the spongioles of the roots, which introduce into the plant the 

 aqueous or lymphatic sap, and the leaves, which elaborate this 

 sap already thickened and enriched, to impel it afterwards to the 

 lower part of the plant, by all the paths that are practicable. 



5. Algce and Mushrooms. 



"We have just presented a rapid picture, or rather given the 

 enumeration, of the different phases of nutrition in a complex 

 plant having roots, stems, and leaves. Naturally things come to 

 pass otherwise in the inferior plants, composed almost wholly of 

 cellular tissues, in what are called in these days Thallophytes, in 

 the algae and mushrooms. 



Here there are neither true vessels nor true roots. The 

 aliments are absorbed by the cells which are in contact with 

 them, and transmitted by endosmosis from point to point. 



Nevertheless the algae have still fellowship with the higher 

 plants by the presence of chlorophyll, even in the coloured algae, 

 where it is masked by colouring matters (nostochineae, and so on). 

 Chlorophyll acts there as it acts everywhere ; it absorbs and 

 assimilates carbon, and only operates in the light. 



In many thallophytes the elementary cell has taken the 

 1 Dutrochet, De VEndosmose, p. 387. 



