THE STRUCTURE AND DEVELOPMENT OF AN UNUSUAL TYPE OF ARTICULATED LATICIFER IN MAMMILLARIA (CACTACEAE)

Although the laticifers of several species of Mammillaria can technically be classified as being of the articulated type, they differ significantly from all other reported articulated laticifers. They are derived from cells which differentiate only in older tissues, never in meristematic or young regions. The development involves the complete lysis of masses of cells, not just the perforation or resorption of the end walls in a single file of cells. At maturity, the laticifer lumen is lined with a one-to-several layered epithelium which may be quite thick. The laticifers increase in diameter with age, apparently by the lysis of the inner epithelial cells. Laticifers occur in the pith, cortex and tubercles of the vegetative body but were not observed in the roots, flower parts or in seedlings up to eight months old. Seven species were studied, all of which have 'milky sap," and the laticifers of each were virtually identical to the laticifers of the others. IT HAS LONG been known that laticifers, or at least cells which produce a "milky sap," occur in certain members of the Cactaceae (Lauterbach, 1889). There has been no detailed study of them, although they have been mentioned by Lauterbach (1889), Craig (1945), Buxbaum (1950), and Boke (1960). While examining the apical meristems of certain species of Mammillaria for another study (1978), I noticed that the anatomy of the laticifers in these plants appeared to be quite different from either of the usual types. An investigation of seven species has shown that the laticifer system in MAammillaria is highly unusual, and although it can be classified as articulated, to my knowledge it is not like any other reported articulated laticifer. MATERIALS AND METHODS-The genus Mammillaria is a large group which contains ca. 320 species (Britton and Rose, 1923; Craig, 1945). They are divided into three groups: 1) those which have "milky sap," 2) those which have "semi-milky sap," and 3) those which have "watery sap." For this study, seven species were selected which all have milky sap. This represents a sample of ca. 6% of the 113 species in this group. The material was fixed in either purified glutaraldehyde (prepared as described previously: Mauseth, 1976) or Randolph's modified Navashin solution (Johansen, 1940). After fixation, the ' Received for publication 2 August 1977; revision accepted 29 November 1977. Support for this work was provided in part by Grant PCM 76-21492 from the National Science Foundation and by Grant 14-7499-0225-URI from The University Research Institute of The University of Texas. material was handled with normal paraffin methods (Johansen, 1940), then stained with Heidenhain's haematoxylin and Orange G (Mauseth, 1976). To avoid tearing the delicate walls which are present at some stages of development, many samples were embedded in Epon plastic, microtomed at I pm on glass knives, and stained with Richardson's stain (Mauseth, 1977). RESULTs-Distribution of laticifers in the plant body-Laticifers occur in both the pith and the cortex of the vegetative shoot, but they have not been found in, or associated with, either the primary or the secondary vascular tissues. The predominant orientation of the laticifers is vertical, but lateral branches which extend through the medullary rays connect the pith laticifers with those of the cortex. The laticifers do not extend to the vicinity of the shoot apical meristem, but terminate, at least in Mammillaria magnimamma, 5,000 ,um below the youngest leaf primordium. The leaves of Mammillaria are extremely reduced and are soon disrupted by tubercle growth (a tubercle is a highly modified leaf base: Buxbaum, 1950; Fig. 1, 2); thus, laticifers do not occur in leaves. Likewise, the roots and flowers (fruits were not available) do not contain laticifers. Seeds and seedlings up to eight months old of Mammillaria compressa, M. brandegeei, and M. geminispina were examined; none had yet developed laticifers. The tubercles are richly supplied with laticifers that connect with those of the cortex (Fig. 4, 5). Because laticifers do not extend into the apical regions of the plant, tubercle primordia and young tubercles are without laticifers. In all cases examined, tracheary elements and spine