THE PROBLEM 53 



Another distinction between the two classes 

 lies in the root-system. In a large proportion 

 of Dicotyledons there is what is called a "tap- 

 root," that is to say, a main root, forming the 

 downward continuation of the stem; this tap- 

 root lasts as long as the stem, and goes on branch- 

 ing, thus supplying all the new roots that are re- 

 quired. In Monocotyledons, there is usually a 

 main root to start with, while the plant is still a 

 seedling, but it soon dies away, and all the sub- 

 sequent roots of the plant spring from the lower 

 part of the stem. This character is correlated 

 with the absence of any growth in thickness, a 

 disability which the roots of Monocotyledons 

 share with the stem. The original root of the 

 seedling is too small to serve for the older plant, 

 and as it cannot grow any stouter it has to be re- 

 placed by successively larger roots, which the 

 stem sends out as it needs them. 



There is no essential distinction between the 

 fl&wers of the two classes, except the curious 

 numerical one (subject, of course, to various ex- 

 ceptions) that the parts of the flower in Mono- 

 cotyledons are usually in threes, while in Dicot- 

 yledons they are usually in fours or fives. In 

 the petrified Cretaceous flower from Japan, re- 

 ferred to above, it was chiefly the fact that there 

 were three carpels which suggested its being a 

 Monocotyledon. 



