ICO Tlie Mode of Examining the [«« ^^^^Cml 



We have now to say a few words on the tissues which remain 

 after the hmitary and fibro-vascular tissues are developed. This 

 mass of cells, part of Naegeli's primitive meristem, Sachs has de- 

 signated Primitive Tissue. Sometimes a considerable quantity of 

 parenchyma is left unaltered, while at other times it can hardly 

 be detected. As, however, in the young state of the stem the 

 primitive tissue can be distinctly seen surrounding the fibro- 

 vascular bundle, and serving, as it were, as a kind of packing, it 

 seems advisable to retain it as a separate tissue. 



The last group of tissues which we have to consider belong to 

 none of the three groups already considered, but while differing from 

 each other morphologically are yet easily classed together physio- 

 logically. Besides, the Laticiferous tissues are not an essential part 

 of the structure of a plant, as they are frequently absent. In 

 examining the laticiferous tissues it is very useful to boil the speci- 

 mens for a short time in dilute solution of caustic potash. This 

 renders the rest of the tissue so transparent that the laticiferous 

 vessels can be seen and traced with great distinctness. Plants 

 belonging to the Cichoraceae, Lobeliaceae, and Campanulacea?, are 

 well adapted for the demonstration of the laticiferous vessels. In 

 the plants belonging to these orders the laticiferous vessels are 

 found in the fibro-vascular bundles, and form an anastomosing 

 network permeating the whole plant. In the Cichoraceae the 

 laticiferous vessels are found in the outer layers of the Phloem part 

 of the fibro-vascular bundles, while in the Campanulacese and 

 Lobeliaceae in the inner layers of the Phloem. In the Papayaceae 

 the laticiferous vessels are only to be found in the Xylem layer of the 

 fibro-vascular bundles. In Papaver, Sanguinaria, Chelidonium, &c., 

 the laticiferous vessels are also very perfectly developed in the 

 Phloem, and singly in the pith and Xylem. They do not anasto- 

 mose freely in the stem, but in the leaves, &c., they form a fine net- 

 work. Laticiferous vessels seem to be formed by the fusion of rows 

 of cells, the transverse partitions breaking down, and a continuous 

 tube being in this way formed. In the Urticaceae, Ficus, and 

 Humulus, the laticiferous vessels are found in the hmitary tissues 

 close to the bast-bundles, and in Ficus also in the pith, but never 

 in the wood. They generally occur as long single tubes, and are 

 not regularly or distinctly branched like the vessels in the Papa- 

 veraceaj and Cichoraceae. In the leaves, however, they seem to 

 anastomose freely. 



In the Euphorbiaceae the laticiferous vessels resemble those of 

 the Urticaceae, but are easily distinguished by the thickness of the 

 walls, which appear in section not unlike bast-fibres. They are 

 developed most completely in the immediate vicinity of the bast- 

 fibres, and run into the bark and pith, forming numerous anasto- 

 moses in the swellings below the leaves. The laticiferous vessels 



