Yukimochi-so

Yukimochi-so is a perennial belonging to the genus Arisaema, which has pseudo dioecism.   It grows up to 25 to 50cm, sometimes reaching about 1m in height.   It grows in the way of sympodial branching with a 2-year growth cycle.   It is endemic to Japan and grows naturally only in some parts of Honshu and the Shikoku region.   Since the main habitat of Yukimochi-so is the Shikoku region, its epithet is Sikokianum.

Yukimochi-so likes rich water and weak light and its preferable habitat is areas under forest that gets little direct sunlight.   It adopts well to this light environment to carry out necessary photosynthesis.   Therefore, when it is exposed to the direct sunlight in the middle of summer, the photoenergy becomes excessive, generating reactive oxygen that degrades chlorophyll or damages the gene that harms the tissue.   Furthermore, the luxuriant foliage in the forests guards the flower of Yukimochi-so from heavy rain.   If the heavy rain pores directly on Yukimochi-so, the spathe collects rain water that damages the flower.

Yukimochi-so sprouts to develop the leaves around the beginning of April.   It develops one leaf consisting of 3 leaflets during the vegetative phase.   When it enters the mature phase, it develops 2 leaves consisting of 3 or 5 leaflets.   After that, it blooms in the middle of April or at the end of April.

The lower half part of the petioles (leaf stalks) of the 2 leaves wraps around the whole circumference of the peduncle (flower stalk) to form the pseudo-stem.   Whereas, its upper half parts develop the leaflets.   This pseudo-stem is wrapped with 6 sheath leaves, some of which often become thin and therefore, fall down when it is flowering time.

Yukimochi-so forms the spadix having one spathe, which characterises the family Araceae.   The spadix normally develops either male florets or female florets.   These florets are usually hidden by the spathe.   The peduncle of the male florets is longer than the one of the female florets.   The male florets are purple owing to its anthers' colour and the female florets are light-green.   The bract is about 7 to 12cm in length.  Its exterior surface is dark purple with white stripes.   Whereas, its interior surface of the upright part is green with white stripes and its interior surface of the lower part, below the opening of the floral chamber, is white.   The top of the appendix is roundly puffed and white in colour.   Capturing this feature, where the top of the appendix is a pure-white oblate sphere, it is named as Yukimochi-so meaning a rice cake as white as snow.

Yukimochi-so produces multiple fruits that closely resemble the fruits of Jack in the pulpit (Arisaema serratum (Thunb.) Schott) in autumn.   It is deep-green in colour at the beginning, but discolours little by little from the end of November to December, turning into a vibrant red in January.   The flower stem turns black after the fructification.  Each fruit contains less than 9 seeds.

Yukimochi-so is dioecism.   However, its gene does not determine its gender (male stock or female stock) but the gender is changed by the plant's size, which characterises the family Arisaema.   When its plant body is small, Yukimochi-so is in the vegetative phase and differentiates and forms only vegetative organs, such as leaves or stems.   At that time, it is asexual.   After that, when the plant body grows larger to some extent, it enters the mature phase to develop male florets.   When it grows even more, it develops female florets.   When it becomes female stock, the total leaves area expands larger and the stock can harvest more product through photosynthesis.   It consequently works to the advantage of its flowerage and fructification.   Whereas, it requires much energy to become the female stock and as a result the number of the stocks that become female is about 10 to 15%.

Yukimochi-so contains saponin and calcium oxalate, mainly in the fruit and the corm, therefore it is poisonous.   Since it produces multiple fruits that resemble short corn and ripen into orangish-red, careful attention is required not to accidentally eat it.

Yukimochi-so is Endangered (EN).   The reduction of satoyama (village-vicinity mountains) as its habitat and the collection being used for a horticultural purpose is a serious problem.   Therefore, it is thought that Yukimochi-so is judged to be at high risk of extinction in the near future.

Classification: Araceae Arisaema
Scientific name: Arisaema sikokianum Franch. & Sav.
Japanese name: Yukimochi-so
English Name: ---
Native locality: The Shikoku region and some parts of Honshu, Japan
Ecological description: Perennial
RDB : Endangered (EN)
Planting place: Gassho-sen
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