T 5 o ROUND THE YEAR 



longed, and then it stands up as a slender wiry stem, 

 clear of all the leaves, and loaded at the summit with 

 spikelets of flowers. The sudden expansion of the 

 haulm is possibly the chief reason for the split leaf- 

 sheath. The sedges, in which the haulm expands and 

 ascends more slowly (the solid pith is one proof of 

 this), have the leaf-sheaths closed. In some small and 

 slender grasses, where the haulm never shoots up to 

 any considerable height, the sheath is closed ; so it is 

 in the Cock's-foot grass, where the haulm dilates only 

 slightly, and remains nearly solid, though here the 

 swelling of the bulky spikelets tears open the upper 

 leaf-sheaths. 



Each section increases in length by growth at its 

 base, and the young and tender tissues above the 

 knots probably derive support from the leaf-sheath 

 which wraps them round. 



The knots bind the fibres closely together. They 

 also stiffen the haulm by forming a diaphragm or floor 

 across the tube. Some authors have doubted whether 

 the mechanical function of the knot in stiffening the 

 tube is of practical importance, but after examin- 

 ing a number of common grasses with special 

 reference to this question, I have no hesitation in 

 saying that the knots do materially stiffen the haulm. 

 They also discharge a very definite and useful function 

 of another kind. They remain capable of absorbing 

 water from the surrounding tissues, and of swelling in 

 consequence. In the uppermost part of the haulm v 

 which is erect, the swelling could produce no" useful 

 effect, and here there are no knots except where the 

 flower-stalks are given off. But at the base 



