THE LACTICIFEROUS VESSELS. 



lines or pores are the parts at which no deposit has occurred. It is still in dispute 



if this deposit has taken place in the spiral direction so commonly 



found in vegetable deposits ; but it is quite certain that in a few 



instances the scalariform has unrolled like a spiral vessel (Fig- 70). 



The use of these vessels differs little, if at all, from that of other 



ducts, viz., that of conveying fluids with rapidity ; but there 



is this great peculiarity, that they are found only in one class of 



plants viz., the ferns (Filices), and there supplant all other forms of 



vascular tissue (Fig. 71). Thus there are two great classes of plants 



which have distinguishing anatomical characters ; viz., the Conifer a ^ 70 _ \ portion of 



or fir tribe, distinguished by its glandular woody fibre, and the a * large scalariform 



fern tribe, known readily by its scalariform tissue. The scala- Jg^gBjSS 



riform tissue is also enduring in a vessel, and showing 



remarkable degree, as was stated of ^fJ^SSS^ 



the glandular woody tissue ; for 

 'ferns, like firs, are abundantly found in the coal measures, 



and Professor Quekett discovered it in a funereal urn dug 



up in the island of Anglesey. 



This appears a favourable point at" which to request 

 the reader to look back and observe the unity of design 

 which appears to pervade the whole structure of plants. 

 We have just seen that there is not, in truth, any essential 

 Fig. 71.-Bundle7f scalariform distinction to be made between the three classes of vas- 

 yesselsinclosed in cellular tis- cular tissue now described spiral vessels, ducts, and scala- 

 L> riform vessels, all of them being composed of a mem- 

 branous tube, with a secondary deposit assuming the spiral 

 direction. It is also evident that these differ in no essential respect from Bothrenchym 

 or pitted tissue ; and from dotted cells and fibre cells, only in size and figure. 

 Thus we have traced the essential identity of the tube with the cell, and of the highly- 

 figured vascular tissue with the simpler cells with a secondary deposit. The woody 

 tissue is, in like manner, an elongated cell of thickened membrane. 



The arrangement or classification of these structures is not as yet in a satisfactory 

 condition, and it is yet a desideratum to find out some general feature by which they may 

 be grouped in a less artificial manner. That one which has already been referred to 

 viz., the simple membrane and the membrane with a secondary deposit as the 

 basis of all tissues, is a step in the right direction. It is clearly xmphilosophical to 

 regard mere markings as points of distinction, where there is not real difference in 

 structure and functions. So far as we have now accompanied our readers there can 

 be no difficulty in acknowledging that we have simply passed through modifications of 

 a simple cell. 



Laticiferous, or Milk-bearing Vessels. There is jet another very inte- 

 resting and somewhat less simple form of tissue to be described viz., the milk-bearing 

 tissue so readily inferred to exist from the white exuding juice of the cut dandelion 

 (Leontodori), and poppy (Papaver], or the yellow juices of the Chelidoniurn. The essential 

 characteristics of this tissue is its branched distribution, and the intermitting or pul- 

 satory motion of its contents. In both these respects it differs from other vegetable 

 tissues, and corresponds very closely with the blood-vessels of animals. It is well 



