COURSE OF THE LATTCIFEROUS TUBES. 433 



rudimentary or very fugacious leaves, and in the Asclepiadese, they branch off at all 

 points from the main trunks, and are distributed through the massive cortical paren- 

 chyma in all directions as far as the epidermis, having an oblique and crooked course. 

 In non-succulent stems with well-developed, persistent foliage leaves, longitudinal 

 tubes occurring in the hypodermal cortical layer, and branched off from the main 

 trunks in the nodes, are, judging from Trecul's statements and the phenomena in 

 Euphorbia to be communicated below, at any rate much more frequent than has 

 been represented by most previous observers. 



In many cases also, branches, which are usually thick tubes, pass from the 

 main trunks into the pith ; in the case of medullary bundles of sieve-tubes, they 

 branch off from the main-stems which accompany the latter (Hoya, Asclepias 

 spec.) In plants without medullary sieve-tubes, as Ficus and Euphorbiae, they arise 

 as branches from the main trunks, principally, but as shown by the succulent 

 Euphorbiae not exclusively, in the nodes. According to the particular case, they are 

 either inserted in the parenchyma throughout the entire thickness of the pith, e. g. 

 Ficus, or they are confined to its periphery, e. g. Euphorbia. 



According to the particular families, and even the species, the general rules stated 

 above are subject to manifold variations. The most important details will be given 

 below, reference being made to Chap. VI, and the special works there cited. Some 

 data referring to the secondary wood and secondary bast, belonging to Chaps. XIV 

 and XV, may here be anticipated. 



I. ARTICULATED LATICIFEROUS TUBES. 



I. CichoriaeeaB. In the stem of those species investigated, which have ordinary col- 

 lateral bundles within the boundary of the plerome, the tubes in the first instance lie around 

 the phloem of each of these bundles. Their longitudinal main trunks here form, as seen 

 in cross section, a single curved row, often interrupted by parenchymatous cells, at the 

 boundary towards the cortical parenchyma ; their numerous transverse anastomoses pass 

 along the outer surface of the approximately semi-cylindrical phloem, which is usually 

 destitute of any sclerenchymatous support. These peripheral tubes are the largest. In 

 the stems of Chondrilla, Taraxacum, Apargia, and Cichorium, no other laticiferous tubes 

 are present. The bundles in the lower part of the stem of Sonchus tenerrimus, Picri- 

 dium tingitanum, and, in a slight degree, of Lactuca virosa, have sieve-tubes on the inner 

 side of the xylem also, and are then accompanied here also by laticiferous tubes, which 

 are connected with those outside by branches passing right round the vascular bundle 

 (Trecul). Finally, where, as in the investigated species of Lactuca, Sonchus, Scor- 

 zonera, Tragopogon, and Hieracium, small medullary bundles of sieve-tubes are present, 

 the latter each contain some laticiferous tubes, which run parallel to the sieve-tubes, and 

 anastomose with one another between them, without however having an open com- 

 munication with the sieve-tubes. According to Trecul, anastomoses take place along 

 the entire internode between the nets of tubes accompanying the different vascular 

 bundles. In all plants of the family in question, they are especially numerous in the 

 nodes, both between the peripheral tubes accompanying the vascular bundles, and be- 

 tween these and the medullary tubes, the latter occurring together with the anastomoses 

 described at p. ■231, between the groups of sieve-tubes belonging to the two regions. 

 In the nodes the nets of laticiferous tubes of the stem are further continued into those 

 of the petioles and of the axillary branches. 



In the petioles and ribs of the leaves, the nets of tubes accompany the vascular 

 bundles with the same arrangement as in the stem, and finally their blanches, which end 



F f 



