Introduction

Considerable interest attaches to the control of growth and development of orchid protocorms and seedlings. A series of experiments were conducted which compared the relative merits of different culture vessels for the germination and growth of seedlings of Cattleya aurantiaca and then attempted to identify some of the changes which occurred in culture vessels during growth.

Materials and methods

Procedure used to surface-sterilize and sow seed

Seed was surface-sterilized using 5% commercial bleach (Domestos: Lever Bros., UK) for 1.5 minutes before sowing onto Thompson's medium (Thompson 1977). All vessels were placed in a Warren Shearer growth cabinet at a fluence rate of 142 μmol m–2 s–1, a temperature of 22.5 ± 2 °C and a relative humidity of 90% (Seaton & Hailes 1989). Continuous light was used unless otherwise stated.

Measurement of growth and development

The percentage of seeds germinated was recorded at 14 days and 28 days, and growth was monitored at intervals by measuring the diameter of 50 protocorms, in each of 4 flasks, using a Leitz inverted microscope equipped with an eyepiece graticule. As this parameter gave no information about the development of protocorms an index of development was also employed, which was modified from that of Spoerl (1948). Seedlings were assigned to one of four different developmental stages (Figure 1). The number in each class was multiplied by the class number, and the values for the different classes summed to give the Protocorm Development Index (PDI).