THE MORPHOLOGICAL COMPOSITION OF PLANTS. 61 



of the solid axis. For while, even more clearly than in the 

 Dendrobium, we see at the point b, a continuity of structure 

 between the substance of the axis below the node, and the 

 substance of the sheath above the node: we see that this 

 sheath, instead of having its edges united as in Dendrobium, 

 has them simply overlapping, so as to form an incomplete 

 hollow cylinder which may be taken off and unrolled; 

 and we see that were the overlapping edges of this sheath 

 united all the way from the node a to the node b, it would 

 constitute a tubular axis, like that which precedes it or like 

 that which it includes. And then, giving an unexpected 

 conclusiveness to the argument, it turns out that in one 

 family of grasses, the overlapping edges of the sheaths do 

 unite : thus furnishing us with a demonstration that tubular 

 structures are produced by the incurving and joining of 

 foliar surfaces; and that so, hollow axes may be interpreted 

 as above, without making any assumption unwarranted by 

 fact. One further correspondence between the type 



thus ideally constructed, and the monocotyledonous type, 

 must be noted. If, as already pointed out, the transverse 

 growth of an axis arises when the axis comes to be a channel 

 of circulation between all the roots at one of its extremities 

 and all the leaves at the other; and if this lateral bulging 

 must increase as fast as the quantity of foliage to be brought 

 in communication with the roots increases — especially if such 

 foliage has at the same time to be raised high above the 

 earth's surface; what must happen to a plant constructed in 

 the manner just described ? The elder fronds or foliar organs, 

 ensheathing the younger ones, as well as the incipient 

 axis serving as a bond of union, are at first of such circum- 

 ference only as suffices to inclose these undeveloped parts. 

 What, then, will take place when the inclosed parts grow — 

 when the axis thickens while it elongates? Evidently the 

 earliest-formed sheaths, not being large enough for the 

 swelling axis, must burst; and evidently each of the later- 

 formed sheaths must, in -its turn, do the like. There must 



