HIPPURIS. STEM. 83 



distribution of tissues is in the main the same as in the 

 internode, but 



1. The large intercellular spaces are divided by horizontal 

 septa, consisting of single layers of cells. 



2. Branch bundles leave the central cylinder, and pass 

 horizontally outwards to the bases of the leaves. 



III. Cut median longitudinal sections of the apical bud of 

 Hippuris, so as to pass through the elongated apical cone ; treat 

 with potash, and mount in dilute glycerine. Examine first with 

 a low power, and observe : 



1. The Axis, which is wide, and cylindrical below, but tapers 

 upwards to the rather elongated apical cone (punctum vege- 

 tationis). The axis is composed of the several tissues already 

 noticed. Note especially : 



(a) The rectangular intercellular spaces divided transversely 

 by septa at the nodes. 



(6) The axial vascular cylinder, which may be followed far 

 up into the apical cone, and which gives out lateral branches to 

 the leaves. 



2. The leaves, diminishing in size towards the apex. Note 

 the scale-hairs about the bases of the leaves. 



Put on a high power, and examine the apical cone. Note : 



1. The Dermatogen, (cf. p. 65) a continuous layer of cells, 

 which covers the apical cone externally. Trace it backwards from 

 the apex : it will be seen to give rise to the epidermis. 



2. The Periblem, consisting of 4-5 layers of cells, which may 

 be traced backwards, and be thus shown to give rise to the 

 cortex. 



3. A central cylinder of Plerome, which is continuous with, 

 and gives rise to, the vascular cylinder. 



Note that the Leaves originate from the outgrowth of the derma- 

 togen and periblem, the plerome taking no part in their formation- 

 Also that the vascular system of the stem is already developed 

 at a higher point on the axis than that of any of the leaves. We 

 have thus an instance of cauline vascular bundles, that is such 

 as are proper to the stem, as distinguished from common 

 vascular bundles, which terminate at their upper extremities 

 in the leaves. 



G 2 



