Sunday, November 1, 2009

The Three Petrocosmea minors - Part 1

A flowering plant of Petrocosmea minor smooth leaf form.

Since first coming into cultivation around 1999, the plant labeled P. minor has been a favorite among Pet admirers. The plant appears to have come into cultivation in the US via an independent vendor from China who sold that plant labeled as P. minor. Almost from the very beginning of it's cultivation, though, the plant's identity as P. minor has been in question. To add to the confusion, in the next few years, new forms of the species began to show up in shows. The plants were very similar in many respects, but had distinct differences in both leaf form, growth habit and flowers. I decided to do a series of posts on the forms of this species that I have in cultivation in my collection.

The flower of the plant I grow as P. minor smooth leaf form. The flowers are borne on single flowered cymes...rarely two-flowered.
Lets begin at the beginning. Initially, there was just the one form of P. minor..the one pictured above, in the US. At the time it was introduced, it was called simply, P. minor, as it came labeled from the vendor. The plants were reported to have been collected in China. Leaves on this form were round...orbicular is shape with smooth dorsal (adaxial) leaf surfaces. The plant habit was flat, symmetrical, and the leaves were distinctly glossy in appearance. Flowers are grape-purple, with a white throat and a deep violet purple almost black spot at the very base of the throat, deep inside the corolla. Cymes or peduncles were single flowered, rarely two-flowered. Cymes have three bracts consistently and calyx lobes are six in number..which is odd for a Petrocosmea where all species except P. kerrii have five calyx lobes. (P. kerrii has three calyx lobes).

The three forms of P. minor...smooth leaf form on the top, veined leaf form on the lower left and pointed leaf form on the lower right. All three of these plants are the same age, in a five inch pan pot, and were grown side by side under the same conditions for one year previous to the photo being taken. P. minor smooth leaf form is intermediate between the two in size of the rosette and has the smallest and glossiest leaves. Veining on the leaves of smooth leaf form is very shallow. Leaves cupped in a convex manner.

A cyme and flower bud of P. minor smooth leaf form, showing the six lobed calyx, with the upper three lobes being shorter than the lower three lobes. Cymes are usually single flowered.

Soon after P. minor was introduced, two other forms became evident. One of these, the form now called P. minor veined leaf form, was originally labeled P. sp. '#5' or 'Chinese #5'. I have grown leaves of this form from various sources, all labeled as #5 and Chinese #5 and have found them all identical to P. minor veined leaf form. The veined leaf form is the largest of the three forms in all respects. The leaves are much heavier in substance, with very deep veining along the dorsal (adaxial) surface, giving it a pebbled appearance to the leaves. It's flowers and cymes are also different from the smooth leaf form. The third form is now called pointed leaf form and I have also seen it labeled P. minor holly leaved form. This form has the smallest rosette of the three forms. It grows much more slowly than the other two forms and has branched cymes with smaller flowers. The leaves on this form fold under at the tip and along the margin to give it a pointed appearance. When the leaves of this form are flattened, however, they are also rounded or orbicular in shape. This form has leaves that are a deeper green color than the other two forms and the leaves are not as glossy, although the dorsal (adaxial) surface remains glabrous or smooth (free of hairs).

Flower cymes of the three forms of P. minor....from left to right...smooth leaf form (left), pointed leaf form (center) and veined leaf form (right). The smooth leaf form consistently has three bracts on the cyme, whereas the pointed leaf form also has secondary bracts and a consistently branched cyme.

The flower structure of the P. minors appears to place them within Section -Anisochilus within the genus Petrocosmea. This section has flowers which have shorter upper lobes...roughly one half the length of the lower petal lobes, and the upper lobes fuse to form a hood or upper lip that has two lobes. This characteristic is also present in the flowers of P. sericea and the P. rosettifolia group.
Culture of the three forms of P. minor is consistent with the culture of all other Pets. Since these are some of the largest of all Pet species, they will need larger diameter pots as they mature. I keep them in five inch diameter x two inch deep pan pots (shallow pots) at maturity, although they would easily grow into larger pots if given them. I find these species quite slow to mature, compared to many other species. I allow them to dry out between waterings. They tolerater drying out better than other species, perhaps because the leaves are of a heavy substance and succulent. In hybridization attempts, I found all of the P. minors to be very difficult to work with. They did not accept pollen from each other, by selfing or from other species. They did cooperate as pollen parents in two hybrids, with P. sericea and P. forrestii.

The last thing to note about these species is that all three forms appear to NOT match the description of P. minor in the Flora of China. Differences in the leaf shape (being round or orbicular in shape), size and texture (being glabrous or smooth on the dorsal leaf surface) as well as the six lobed calyx, and the shape of the filaments, cause them not to match any currently described species description within the Flora of China in my opinion. I have studied the flowers, cymes, and leaves under a microscope and carefully compared them to the description within the Flora of China and cannot find a match among the twenty-plus descriptions of Petrocosmea species contained therein.
So, even though these beautiful Petrocosmea species are currently suffering a bit from an identity crisis, they remain among the most unique and beautiful Pets in my collection.
Stay tuned for Part 2 and Part 3 of this series on P. minor!!!